Add new "exec_once_unless_exception" system; apply to dialect.initialize

Add new "exec_once_unless_exception" system; apply to dialect.initialize

Fixed an issue whereby if the dialect "initialize" process which occurs on
first connect would encounter an unexpected exception, the initialize
process would fail to complete and then no longer attempt on subsequent
connection attempts, leaving the dialect in an un-initialized, or partially
initialized state, within the scope of parameters that need to be
established based on inspection of a live connection.   The "invoke once"
logic in the event system has been reworked to accommodate for this
occurrence using new, private API features that establish an "exec once"
hook that will continue to allow the initializer to fire off on subsequent
connections, until it completes without raising an exception. This does not
impact the behavior of the existing ``once=True`` flag within the event
system.

Fixes: #4807
Change-Id: Iec32999b61b6af4b38b6719e0c2651454619078c