Record the last time a connection was used on the connection object

Summary:
Ref T11458. Currently, we're very aggressive about closing connections.

I want to change things so we only close connections we haven't used recently (for example: no activity in the last 15 seconds). This should let us do a better job of holding open approximately only the connections we actually need in a wider range of cases.

Test Plan:
Ran some code that uses queries.

This doesn't do anything on its own; see next diff in Phabricator for meaningful changes.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11458

Differential Revision: https://secure.phabricator.com/D16388