Timeout and check on write

[mrocklin]

The core.write coroutine waits on IOStream.write futures. However, there are
some cases where IOStream.write futures don't complete. Notably if something
else writes to the same stream before we're done waiting then the first future
is forgotten and hangs forever.

If `write` is called again before that `.Future` has resolved, the
previous future will be orphaned and will never resolve.

This solution wraps the wait in a timeout and checks the stream's
write_buffer as a secondary check for completion. There are probably better
solutions that don't rely on the timeout but this should work fine.

cc @pitrou