TwoWayPipe and basic asyncio support

Note: I've tried not to break any behaviour of the previously working APIs

Python API Changes / Additions
- capnp/lib/capnp.pyx
  * class _RemotePromise
    + [Added] cpdef _wait(self)
      = Exception raising code that used to be inside of wait(self)
    + [Modified] def wait(self)
      = Same functionality as before
    + [Added] async def a_wait(self)
      = Cannot use await as that's a reserved keyword
      = Uses pollRemote and asyncio.sleep(0) to make call asynchronous
  * class _TwoPartyVatNetwork
    + [Added] cdef _init_pipe(self, _TwoWayPipe pipe, Side side,
    schema_cpp.ReaderOptions opts)
      = Instanciates a TwoPartyVatNetwork using a TwoWayPipe (instead of
      using a file handle or connection as before)
  * class TwoPartyClient
    + [Modified] def __init__(self, socket=None, restorer=None,
    traversal_limit_in_words=None, nesting_limit=None)
      = Changes the socket parameter to be optional
      = If socket is not specified, default to using a TwoWayPipe
    + [Added] async def read(self, bufsize)
      = awaitable function that blocks until data has been read
      = bufsize defines the maximum amount of data to be read back
        (e.g. 4096 bytes)
      = Reads data from TwoWayPipe
    + [Added] def write(self, data)
      = Write data to TwoWayPipe
      = Not awaitable as the write interface of the TwoWayPipe doesn't
      have poll functionality
  * class TwoPartyServer
    + [Modified] def __init__(self, socket=None, restorer=None,
    server_socket=None, bootstrap=None, traversal_limit_in_words=None,
    nesting_limit=None)
      = Changes the socket parameter to be optional
      = If socket is not specified, default to using a TwoWayPipe
      = Simplified code by removing an else (self._connect)
    + [Added] async def read(self, bufsize)
      = awaitable function that blocks until data has been read
      = bufsize defines the maximum amount of data to be read back
        (e.g. 4096 bytes)
      = Reads data from TwoWayPipe
    + [Added] def write(self, data)
      = Write data to TwoWayPipe
      = Not awaitable as the write interface of the TwoWayPipe doesn't
      have poll functionality
    + [Added] async def poll_forever(self)
      = asyncio equivalent of run_forever()
  * class _TwoWayPipe
    + Wrapper class for TwoWayPipe

Other Additions
- capnp/helpers/asyncHelper.h
  * pollWaitScope
    + Pumps the kj event handler
    + Used for the TwoWayServer
  * pollRemote
    + Polls a remote promise
    + i.e. a capnp RPC call
- capnp/helpers/asyncIoHelper.h
  * AsyncIoStreamReadHelper
    + I wasn't able to figure out Promise[size_t] using Cython so this was
    the next best thing I could think of doing
    + Was needed to handle read polling from a read promise
      = Polling is used for asyncio as kj waits need a wrapper to be
      compatible
- capnp/lib/capnp.pyx
  * makeTwoWayPipe
    + Wrapper for kj newTwoWayPipe function
  * poll_once
    + Single pump of the kj event handler (used with pollWaitScope)

TwoWayClient Usage - TwoWayPipe
- See examples/async_client.py

TwoWayServer Usage - TwoWayPipe
- See examples/async_server.py

capnp/helpers/asyncIoHelper.h

Misc Changes
- Fixed thread_server.py and thread_client.py to use bootstrap instead
of ez_restore
- async_client.py and async_server.py examples
  * Uses the same thread.capnp as thread_client.py and thread_server.py
  * They are compatible, so you can mix and match client and server for
  compatibility testing
  * async_client.py and async_server.py require <address>:<port>
  formatting (unlike autodetection from thread_client.py and
  thread_server.py)

examples/async_server.py

@@ -0,0 +1,87 @@
+#!/usr/bin/env python
+
+from __future__ import print_function
+
+import argparse
+import capnp
+
+import thread_capnp
+import asyncio
+import socket
+
+
+class ExampleImpl(thread_capnp.Example.Server):
+
+    "Implementation of the Example threading Cap'n Proto interface."
+
+    def subscribeStatus(self, subscriber, **kwargs):
+        return capnp.getTimer().after_delay(10**9) \
+            .then(lambda: subscriber.status(True)) \
+            .then(lambda _: self.subscribeStatus(subscriber))
+
+    def longRunning(self, **kwargs):
+        return capnp.getTimer().after_delay(3 * 10**9)
+
+
+async def myreader(server, reader):
+    while True:
+        data = await reader.read(4096)
+        # Close connection if 0 bytes read
+        if len(data) == 0:
+            server.close()
+        await server.write(data)
+
+
+async def mywriter(server, writer):
+    while True:
+        data = await server.read(4096)
+        writer.write(data.tobytes())
+        await writer.drain()
+
+
+async def myserver(reader, writer):
+    # Start TwoPartyServer using TwoWayPipe (only requires bootstrap)
+    server = capnp.TwoPartyServer(bootstrap=ExampleImpl())
+
+    # Assemble reader and writer tasks, run in the background
+    coroutines = [myreader(server, reader), mywriter(server, writer)]
+    asyncio.gather(*coroutines, return_exceptions=True)
+
+    await server.poll_forever()
+
+
+def parse_args():
+    parser = argparse.ArgumentParser(usage='''Runs the server bound to the\
+given address/port ADDRESS. ''')
+
+    parser.add_argument("address", help="ADDRESS:PORT")
+
+    return parser.parse_args()
+
+
+async def main():
+    address = parse_args().address
+    host = address.split(':')
+    addr = host[0]
+    port = host[1]
+
+    # Handle both IPv4 and IPv6 cases
+    try:
+        print("Try IPv4")
+        server = await asyncio.start_server(
+            myserver,
+            addr, port,
+        )
+    except:
+        print("Try IPv6")
+        server = await asyncio.start_server(
+            myserver,
+            addr, port,
+            family=socket.AF_INET6
+        )
+
+    async with server:
+        await server.serve_forever()
+
+if __name__ == '__main__':
+    asyncio.run(main())