Prevent double-checkins and guard during reset-on-return invalidations

Prevent double-checkins and guard during reset-on-return invalidations

Fixed connection pool issue whereby if a disconnection error were raised
during the connection pool's "reset on return" sequence in conjunction with
an explicit transaction opened against the enclosing :class:`.Connection`
object (such as from calling :meth:`.Session.close` without a rollback or
commit, or calling :meth:`.Connection.close` without first closing a
transaction declared with :meth:`.Connection.begin`), a double-checkin would
result, which could then lead towards concurrent checkouts of the same
connection. The double-checkin condition is now prevented overall by an
assertion, as well as the specific double-checkin scenario has been
fixed.

Change-Id: If5bb6941e36326846b14918c33ebfdd5604f642e
Fixes: #4252