Tweak get_connection_info() and keepalive_expiry behaviour.

[tomchristie]

There are two changes here...

stats = pool.get_connection_info()

Instead becomes...

stats = await pool.get_connection_info()

This is important because it allows us to run a keep-alive expiry sweep before returning the connection info, so you'll be seeing more valid information.

Secondly, we were previously capping our keep-alive expiry checks to at most once per second. Now if keepalive_expiry is set, and is less than 1.0, then we'll check for expiry more frequently than once per second. In particular this ensures that keepalive_expiry=0.0 functions as expected.

Note that .get_connection_info() is only introduced from 0.10 onwards, so this is an addition, not a breaking change.