Invalidate on failed connect handler

Invalidate on failed connect handler

Fixed bug in connection pool where a connection could be present in the
pool without all of its "connect" event handlers called, if a previous
"connect" handler threw an exception; note that the dialects themselves
have connect handlers that emit SQL, such as those which set transaction
isolation, which can fail if the database is in a non-available state, but
still allows a connection.  The connection is now invalidated first if any
of the connect handlers fail.

Change-Id: I61d6f4827a98ab8455f1c3e1c55d046eeccec09a
Fixes: #4225