for better separation of concerns split out the
core pooling constructs from the implementations.  also
makes more room for new pool classes should we decide to
add them.  The DBProxy feature is never used so remove it
from documentation and split it out.

Change-Id: I04c9d7ae1882b17f3cf5c37ed05b781b05ef88fa
Fixes: #4255

Fixed connection pool issue whereby if a disconnection error were raised
during the connection pool's "reset on return" sequence in conjunction with
an explicit transaction opened against the enclosing :class:`.Connection`
object (such as from calling :meth:`.Session.close` without a rollback or
commit, or calling :meth:`.Connection.close` without first closing a
transaction declared with :meth:`.Connection.begin`), a double-checkin would
result, which could then lead towards concurrent checkouts of the same
connection. The double-checkin condition is now prevented overall by an
assertion, as well as the specific double-checkin scenario has been
fixed.

Change-Id: If5bb6941e36326846b14918c33ebfdd5604f642e
Fixes: #4252

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

Fixed a fairly serious connection pool bug where a connection that is
acquired after being refreshed as a result of a user-defined
:class:`.DisconnectionError` or due to the 1.2-released "pre_ping" feature
would not be correctly reset if the connection were returned to the pool by
weakref cleanup (e.g. the front-facing object is garbage collected); the
weakref would still refer to the previously invalidated DBAPI connection
which would have the reset operation erroneously called upon it instead.
This would lead to stack traces in the logs and a connection being checked
into the pool without being reset, which can cause locking issues.

Change-Id: Iabd9f3a63a1d0207d0de0054a6ced3560818cf9c
Fixes: #4184

This test often fails on CI under load and reveals a small
race condition within SingleonThreadPool's "cleanup" system.
As this pool's "cleanup" system is pretty much never used in
any case since this pool is no longer used for SQLite file
databases, this race should have little impact, especially
since the occasional non-closed connection will get closed
out when it's GC'ed in any case.

Change-Id: I68aefc0f9bbfc708c95af0497844e65366fc8429

now and it seems like mock might not be doing the right thing.

Change-Id: I5c108d82631c9217da54a8ace68d7728c3e204d8

used in this intentionally broken scenario

Change-Id: I88ea6fa710da2189e6d47e2d12f5f0fd6f6bb7d4

- 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

Change-Id: Ie4cb4924909d55c5962f66e36cd5325e8e8f0538