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)

capnp/helpers/asyncIoHelper.h

@@ -0,0 +1,53 @@
+#pragma once
+
+#include "kj/async.h"
+#include "kj/async-io.h"
+
+class AsyncIoStreamReadHelper {
+public:
+  AsyncIoStreamReadHelper(kj::AsyncIoStream * _stream, kj::WaitScope * _scope, size_t bufsize) {
+    io_stream = _stream;
+    wait_scope = _scope;
+    ready = false;
+    buffer_read_size = 0;
+    buffer = new unsigned char[bufsize];
+    promise = io_stream->read(buffer, 1, bufsize);
+  }
+
+  ~AsyncIoStreamReadHelper() {
+    delete[] buffer;
+  }
+
+  bool poll() {
+    bool result = promise.poll(*wait_scope);
+    if (result) {
+      ready = true;
+      buffer_read_size = promise.wait(*wait_scope);
+    }
+    return result;
+  }
+
+  size_t read_size() {
+    if (!ready) {
+      return 0;
+    }
+    return buffer_read_size;
+  }
+
+  void * read_buffer() {
+    if (!ready) {
+      return 0;
+    }
+    return buffer;
+  }
+
+private:
+  kj::AsyncIoStream * io_stream;
+  kj::WaitScope * wait_scope;
+  kj::Promise<size_t> promise = nullptr;
+
+  unsigned char *buffer;
+  size_t buffer_read_size;
+
+  bool ready;
+};