Asyncio drain fix

[Askaholic]

The server logs are plagued with the following error:

Traceback (most recent call last):
  File "/code/server/__init__.py", line 148, in do_report_dirties
    await asyncio.gather(*tasks)
  File "/code/server/servercontext.py", line 56, in broadcast_raw
    await asyncio.gather(*tasks)
  File "/code/server/protocol/qdatastreamprotocol.py", line 150, in send_raw
    await self.writer.drain()
  File "/usr/local/lib/python3.6/asyncio/streams.py", line 339, in drain
    yield from self._protocol._drain_helper()
  File "/usr/local/lib/python3.6/asyncio/streams.py", line 214, in _drain_helper
    assert waiter is None or waiter.cancelled()
AssertionError

It appears that this is caused by drain being awaited by multiple coroutines. As a result this is non-trivial to reproduce on the test server as it requires a lot of messages to be sent at the same time. As far as I can tell this behavior is considered a bug in asyncio, as it is not mentioned in the documentation. A workaround, which is employed by the websockets project, is to serialize the calls to drain with a mutex.

I don't expect the addition of a lock to have a noticeable effect on performance as typically the calls to drain were intended to serve as a way of serializing messages per connection anyways (applying backpressure), the only place where this wouldn't naturally occur was in the broadcast functions, which are the reason for occasionally getting concurrent calls to drain.

[Wesmania]

Looks fine to me