Allow 0-RTT data to be coalesced with the INITIAL using asyncio API

Make it possible for `connect` method to not send any data, in order to
allow applications to make use of 0-RTT.

Fixes: #492.

src/aioquic/asyncio/client.py

@@ -44,6 +44,11 @@ async def connect(
     * ``stream_handler`` is a callback which is invoked whenever a stream is
       created. It must accept two arguments: a :class:`asyncio.StreamReader`
       and a :class:`asyncio.StreamWriter`.
+    * ``wait_connected`` indicates whether the context manager should wait for the
+      connection to be established before yielding the
+      :class:`~aioquic.asyncio.QuicConnectionProtocol`. By default this is `True` but
+      you can set it to `False` if you want to immediately start sending data using
+      0-RTT.
     * ``local_port`` is the UDP port number that this client wants to bind.
     """
     loop = asyncio.get_event_loop()
@@ -83,7 +88,7 @@ async def connect(
     )
     protocol = cast(QuicConnectionProtocol, protocol)
     try:
-        protocol.connect(addr)
+        protocol.connect(addr, transmit=wait_connected)
         if wait_connected:
             await protocol.wait_connected()
         yield protocol

src/aioquic/asyncio/protocol.py

@@ -64,14 +64,15 @@ def close(
         )
         self.transmit()
 
-    def connect(self, addr: NetworkAddress) -> None:
+    def connect(self, addr: NetworkAddress, transmit=True) -> None:
         """
         Initiate the TLS handshake.
 
         This method can only be called for clients and a single time.
         """
         self._quic.connect(addr, now=self._loop.time())
-        self.transmit()
+        if transmit:
+            self.transmit()
 
     async def create_stream(
         self, is_unidirectional: bool = False