Fixed fairly critical issue where the DBAPI connection could be returned to
the connection pool while still in an un-rolled-back state. The reset agent
responsible for rolling back the connection could be corrupted in the case
that the transaction was "closed" without being rolled back or committed,
which can occur in some scenarios when using ORM sessions and emitting
.close() in a certain pattern involving savepoints.   The fix ensures that
the reset agent is always active.

note that the reset agent will go away in 2.0 and the only real
purpose of it is for logging of ROLLBACK.   Apparently with the
SQLite singleton engine in the test suite, there are some strucutral
mismatches in the test fixtures where the reset agent is getting
set differently than the transaction likely due to the same connection
being shared in multiple context, though it's unclear.

Fixes: #5326
Change-Id: If056870ea70a2d9a1749768988d5e023f3061b31

includes more replacements for create_engine(), Connection,
disambiguation of Result from future/baked

Change-Id: Icb60a79ee7a6c45ea9056c211ffd1be110da3b5e

Replaces a wide array of Sphinx-relative doc references
with an abbreviated absolute form now supported by
zzzeeksphinx.

Change-Id: I94bffcc3f37885ffdde6238767224296339698a2

Applied an explicit "cause" to most if not all internally raised exceptions
that are raised from within an internal exception catch, to avoid
misleading stacktraces that suggest an error within the handling of an
exception.  While it would be preferable to suppress the internally caught
exception in the way that the ``__suppress_context__`` attribute would,
there does not as yet seem to be a way to do this without suppressing an
enclosing user constructed context, so for now it exposes the internally
caught exception as the cause so that full information about the context
of the error is maintained.

Fixes: #4849
Change-Id: I55a86b29023675d9e5e49bc7edc5a2dc0bcd4751

The pool "pre-ping" feature has been refined to not invoke for a DBAPI
connection that was just opened in the same checkout operation.  pre ping
only applies to a DBAPI connection that's been checked into the pool
and is being checked out again.

Fixes: #4524
Change-Id: Ibe3dfb709dbdc24aa94e96513cfbea456c33b895

This collection was added only for the benefit of unit tests
and is unnecessary for the pool to function.  As SQLAlchemy 2.0
will be removing the automatic handling of connections that are
garbage collection, remove this collection so that we ultimately
don't need a weakref handler to do anything within the pool.
The handler will do nothing other than emit a warning that
a connection was dereferenced without being explicitly returned
to the pool, invalidated, or detached.

Change-Id: I4ca196270d5714efbac44dbf6f034e8c7f0af58a

sqlalchemy.sql.naming was causing a full import of
engine due to the DDLEvents dependency.  Break out pool,
DDL and engine events into new modules specific to those
packages; resolve some other import cycles in Core also.

Change-Id: Ife8d217e58a26ab3605dd80ee70837968f957eaf

(zzzeek:) For some reason I thought that threading.Lock() still did
not support context managers, at least in Python 2, however this
does not seem to be the case.

Co-authored-by: Mike Bayer <mike_mp@zzzcomputing.com>
Closes: #5069
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/5069
Pull-request-sha: efeac06dda5afdbe33abcf9b27c8b5b5725c8444

Change-Id: Ic64fcd99cd587bc70b4ecc5b45d8205b5c76eff2

Change-Id: I08440dc25e40ea1ccea1778f6ee9e28a00808235