protocol: add a lock around StreamWriter.drain(), closes #16

Works around the following error:

    File /usr/lib/python3.4/asyncio/streams.py, line 194, in _drain_helper
      assert waiter is None or waiter.cancelled()
    AssertionError

when the write buffer reaches the high watermark (and thus blocks) and
2+ tasks try to call drain().  Clearly, asyncio's current code (all
versions up to 3.6.1 included) is not "thread" safe.  So use a lock
around the only place drain() is called.

websockets/protocol.py

@@ -124,6 +124,7 @@ def __init__(self, *,
 
         self.reader = None
         self.writer = None
+        self._drain_lock = asyncio.Lock(loop=loop)
 
         self.request_headers = None
         self.raw_request_headers = None
@@ -562,8 +563,12 @@ def write_frame(self, opcode, data=b''):
                     yield
 
         try:
-            # Handle flow control automatically.
-            yield from self.writer.drain()
+            # drain() cannot be called concurrently by multiple coroutines:
+            # http://bugs.python.org/issue29930. Remove this lock when no
+            # version of Python where this bugs exists is supported anymore.
+            with (yield from self._drain_lock):
+                # Handle flow control automatically.
+                yield from self.writer.drain()
         except ConnectionError:
             # Terminate the connection if the socket died.
             yield from self.fail_connection(1006)