Found using: https://github.com/intgr/topy

1. make sure pool._invalidate() sets the timestamp up before
invalidating the target connection.  we can otherwise show how the
conn.invalidate() + pool._invalidate() can lead to an extra connection
being made.
2. to help with that, soften up the check on connection.invalidate()
when connection is already closed. a warning is fine here
3. add a mutex to test_max_overflow() when we connect, because the way
we're using mock depends on an iterator, that needs to be synchronized

- move out logging tests from test_execute to test_logging

implementation allows an event handler to redefine the specific mechanics
by which an arbitrary dialect invokes execute() or executemany() on a
DBAPI cursor.  The new events, at this point semi-public and experimental,
are in support of some upcoming transaction-related extensions.

after one or more :class:`.Connection` objects have been created
(such as by an orm :class:`.Session` or via explicit connect)
and the listener will pick up events from those connections.
Previously, performance concerns pushed the event transfer from
:class:`.Engine` to  :class:`.Connection` at init-time only, but
we've inlined a bunch of conditional checks to make this possible
without any additional function calls. fixes #2978