Tests illustrate that exceptions like GreenletExit and
even KeyboardInterrupt can corrupt the state of a DBAPI
connection like that of pymysql and mysqlclient.   Intercept
BaseException errors within the handle_error scheme and
invalidate just the connection alone in this case, but not
the whole pool.

The change is backwards-incompatible with a program that
currently intercepts ctrl-C within a database transaction
and wants to continue working on that transaction.  Ensure
the event hook can be used to reverse this behavior.

Change-Id: Ifaa013c13826d123eef34e32b7e79fff74f1b21b
Fixes: #3803

- add a connect=True key to connection record to support
  pre-loading of _ConnectionRecord objects
- ensure _ConnectionRecord.close() leaves the record in a good
  state for reopening
- add _ConnectionRecord.record_info for persistent storage
- add "in_use" accessor based on fairy_ref being present or not
- allow for the exclusions system and SuiteRequirements to be
  usable without the full plugin_base setup.
- move some Python-env requirements to the importable
  requirements.py module.
- allow starttime to be queried
- add additional events for engine plugins
- have "dialect" be a first-class parameter to the pool,
  ensure the engine strategy supplies it up front

Change-Id: Ibf549f7a1766e49d335cd6f5e26bacfaef9a8229

has no return value

:meth:`.ConnectionEvents.detach`,
:meth:`.ConnectionEvents.close_detached`.

may be called against a stale connection without the "connect"
event handler having been called, in the case where the pool
attempted to reconnect after being invalidated and failed; the stale
connection would remain present and would be used on a subsequent
attempt.  This issue has a greater impact in the 1.0 series subsequent
to 1.0.2, as it also delivers a blanked-out ``.info`` dictionary to
the event handler; prior to 1.0.2 the ``.info`` dictionary is still
the previous one.
fixes #3497

and the database can no longer be connected towards, that the checkout
handler failure is caught, the attempt to re-acquire the connection
also raises an exception, but the underlying connection record
is not immediately re-checked in before the exception is propagated
outwards, having the effect that the checked-out record does not close
itself until the stack trace it's associated with is garbage collected,
preventing that record from being used for a new checkout until we
leave the scope of the stack trace.   This can lead to confusion
in the specific case of when the number of current stack traces
in memory exceeds the number of connections the pool can return,
as the pool will instead begin to raise errors about no more checkouts
available, rather than attempting a connection again.   The fix
applies a checkin of the record before re-raising.
fixes #3419

using descriptors; ensure that mock.patch() honors descriptor setters

old / new style, direct access, and ad-hoc patching and
unpatching as possible