pep-484 for pool

pep-484 for pool

also extends into some areas of utils, events and others
as needed.

Formalizes a public hierarchy for pool API,
with ManagesConnection -> PoolProxiedConnection /
ConnectionPoolEntry for connectionfairy / connectionrecord,
which are now what's exposed in the event API and other
APIs.  all public API docs moved to the new objects.

Corrects the mypy plugin's check for sqlalchemy-stubs
not being insatlled, which has to be imported using the
dash in the name to be effective.

Change-Id: I16c2cb43b2e840d28e70a015f370a768e70f3581