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

Raise a more descriptive exception / message when ClauseElement
or non-SQLAlchemy objects that are not "executable" are erroneously
passed to ``.execute()``; a new exception ObjectNotExecutableError
is raised consistently in all cases.

Change-Id: I2dd393121e2c7e5b6b9e40286a2f25670876e8e4
Fixes: #3786

via :paramref:`.create_engine.isolation_level` and
:paramref:`.Connection.execution_options.isolation_level`
parameters.  fixes #3534

back by using an attrgetter for the default case

have it; keep it simple