Exception on timeout from connection pool acquire

[masell]

• asyncpg version: v0.20.0
• PostgreSQL version: 11
• Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
  the issue with a local PostgreSQL install?:
• Python version: 3.7
• Platform: Ubuntu
• Do you use pgbouncer?: No
• Did you install asyncpg with pip?: Yes
• If you built asyncpg locally, which version of Cython did you use?:
• Can the issue be reproduced under both asyncio and
  uvloop?: Idk, probably.

This one fixes timeout error
#468

This one line breaks it.

bdba7ce#diff-8424be9bb2792628343ee98369a5cf74L529

22:13:29 ERROR [/usr/lib/python3.7/asyncio/base_events.py:1608] Task exception was never retrieved
future: <Task finished coro=<Pool._acquire.<locals>._acquire_impl() done, defined at .venv/lib/python3.7/site-packages/asyncpg/pool.py:595> exception=AttributeError("'coroutine' object has no attribute 'add_done_callback'")>
Traceback (most recent call last):
  File ".venv/lib/python3.7/site-packages/asyncpg/connect_utils.py", line 621, in _connect_addr
    connector, timeout=timeout)
  File "/usr/lib/python3.7/asyncio/tasks.py", line 409, in wait_for
    await waiter
concurrent.futures._base.CancelledError

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File ".venv/lib/python3.7/site-packages/asyncpg/pool.py", line 598, in _acquire_impl
    proxy = await ch.acquire()  # type: PoolConnectionProxy
  File ".venv/lib/python3.7/site-packages/asyncpg/pool.py", line 133, in acquire
    await self.connect()
  File ".venv/lib/python3.7/site-packages/asyncpg/pool.py", line 125, in connect
    self._con = await self._pool._get_new_connection()
  File ".venv/lib/python3.7/site-packages/asyncpg/pool.py", line 472, in _get_new_connection
    **self._connect_kwargs)
  File ".venv/lib/python3.7/site-packages/asyncpg/connection.py", line 1677, in connect
    max_cacheable_statement_size=max_cacheable_statement_size)
  File ".venv/lib/python3.7/site-packages/asyncpg/connect_utils.py", line 654, in _connect
    connection_class=connection_class)
  File ".venv/lib/python3.7/site-packages/asyncpg/connect_utils.py", line 623, in _connect_addr
    connector.add_done_callback(_close_leaked_connection)
AttributeError: 'coroutine' object has no attribute 'add_done_callback'