Move begin ping listener to a connect listener

The "reconnect" listener that occurs on transaction begin
is not sufficient for libraries that use engines, connections
and sessions in "autocommit" mode, where no transaction is begun.
So we move our "ping" from the begin phase to the "connect"
phase instead.   We use a newer SQLAlchemy event "engine_connect"
so that we can make use of the Connection and all the disconnect/
invalidation facilities that are included, rather than using
the low-level pool listener which is somewhat legacy.
Since the event was added in SQLAlchemy 0.9.0, a compatibility
layer is added which emulates the mechanics of the
handle_error compatibility layer.

Change-Id: I719fe45e657e747e5f8d64f20059e2b61c79b63d
Closes-bug: #1374497
Merge "Move begin ping listener to a connect listener"