drop a 400 ton anvil on oracle 23c

drop a 400 ton anvil on oracle 23c

this DB is extremely erratic in being able to connect.   Add
a brute force connection retrier to all engines everywhere
(which for oracledb we can fortunately use their built-in feature
that also works).
This actually works and I can see it pausing under load, reconnecting,
and succeeding.  the problem is that absolutely every engine everywhere
needs this routine otherwise an engine without a retrier in it will
crash.    That then necessitates digging into testing_engine(),
making sure testing_engine() is used everywhere an engine that's going
to connect is used, then dealing with the fallout from that.

We also simplify some older workarounds for cx_oracle and
hack into config/provision to make oracledb seem like the primary
DBAPI for most tests.

testing_engine has been completely overhauled, making use of a new
post_configure_testing_engine() hook which moves and refines
the SQLite pool sharing and savepoint logic all into sqlite/provision.py
and also allows for cx_oracle to apply a retry event handler.

Change-Id: I4ea4c523effb878290d28b94d8925eb32fc5ae3b