Called after the :meth:`.Engine.dispose` method is called.

:func:`.engine_from_config` were not being parsed correctly;
these included ``pool_threadlocal`` and the psycopg2 argument
``use_native_unicode``. fixes #3435
- add legacy_schema_aliasing config parsing for mssql
- move use_native_unicode config arg to the psycopg2 dialect

the URL design a little simpler

:meth:`.URL.get_dialect` method will continue to return the
ultimate :class:`.Dialect` object when a dialect plugin is used,
without the need for the caller to be aware of the
:meth:`.Dialect.get_dialect_cls` method.
reference #3379

pep-249 exception names linked to exception classes of an entirely
different name, preventing SQLAlchemy's own exception wrapping from
wrapping the error appropriately.
The SQLAlchemy dialect in use needs to implement a new
accessor :attr:`.DefaultDialect.dbapi_exception_translation_map`
to support this feature; this is implemented now for the py-postgresql
dialect.
fixes #3421

functionality.   Added a new "soft invalidate" feature to the
connection pool at the level of the checked out connection wrapper
as well as the :class:`._ConnectionRecord`.  This works similarly
to a modern pool invalidation in that connections aren't actively
closed, but are recycled only on next checkout; this is essentially
a per-connection version of that feature.  A new event
:class:`.PoolEvents.soft_invalidate` is added to complement it.
fixes #3379

- Added new flag
:attr:`.ExceptionContext.invalidate_pool_on_disconnect`.
Allows an error handler within :meth:`.ConnectionEvents.handle_error`
to maintain a "disconnect" condition, but to handle calling invalidate
on individual connections in a specific manner within the event.

- Added new event :class:`.DialectEvents.do_connect`, which allows
interception / replacement of when the :meth:`.Dialect.connect`
hook is called to create a DBAPI connection.  Also added
dialect plugin hooks :meth:`.Dialect.get_dialect_cls` and
:meth:`.Dialect.engine_created` which allow external plugins to
add events to existing dialects using entry points.
fixes #3355

fail to store the correct value for MSSQL on an INSERT where the
primary key value was present in the insert params before execution.
fixes #3360

That is, after exhausing all rows using the fetch methods, the
DBAPI cursor is released as before and the object may be safely
discarded, but the fetch methods may continue to be called for which
they will return an end-of-result object (None for fetchone, empty list
for fetchmany and fetchall).   Only if :meth:`.ResultProxy.close`
is called explicitly will these methods raise the "result is closed"
error.
fixes #3330 fixes #3329

persistence.py could theoretically hit the limit of the cache
(100 items by default) and at some points fail to have a key that
we check for, due to the cleanup.   This has never been observed
so its likely that so far, the total number of INSERT, UPDATE and
DELETE statement structures in real apps has not exceeded 100
on a per-mapper basis; this could happen for apps that run a
very wide variety of attribute modified combinations into the unit
of work, *and* which have very high concurrency going on.
This change will be a lot more significant when we open up
use of LRUCache + compiled cache with the baked query extension.