Adding pure python SSL test using asyncio

- Uses thread.capnp
- Follows same format as thread_client.py/thread_server.py and
async_client.py/async_server.py
- Including a basic self signed certificate for testing convenience
- Python 3.7 has a bug cleaning up SSL when using asyncio.run
  https://bugs.python.org/issue36709
  Have a slightly more verbose workaround to do proper cleanup

examples/async_ssl_server.py

@@ -0,0 +1,94 @@
+#!/usr/bin/env python
+
+from __future__ import print_function
+
+import argparse
+import capnp
+
+import thread_capnp
+import asyncio
+import socket
+import ssl
+
+
+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]
+
+    # Setup SSL context
+    ctx = ssl.create_default_context(ssl.Purpose.CLIENT_AUTH)
+    ctx.load_cert_chain('selfsigned.cert', 'selfsigned.key')
+
+    # Handle both IPv4 and IPv6 cases
+    try:
+        print("Try IPv4")
+        server = await asyncio.start_server(
+            myserver,
+            addr, port,
+            ssl=ctx,
+        )
+    except:
+        print("Try IPv6")
+        server = await asyncio.start_server(
+            myserver,
+            addr, port,
+            ssl=ctx,
+            family=socket.AF_INET6,
+        )
+
+    async with server:
+        await server.serve_forever()
+
+if __name__ == '__main__':
+    asyncio.run(main())