AsyncConnectionPool.open() made async.

AsyncConnectionPool.open() made async.

This makes it symmetrical with close(). However it doesn't really do any
async work as it's awkward to call it from init. Something we might do,
if that will be needed, could be to start the scheduler only and use it
to schedule immediately a call to an async _open().

In the future, an anyio-based pool might instead disallow open=True on
init.

See https://github.com/psycopg/psycopg/pull/151 for some discussion
about the topic.