Ensure extended instrumentation is fully disposed

Ensure extended instrumentation is fully disposed

Fixed regression in the ``sqlalchemy.ext.instrumentation`` extension that
prevented instrumentation disposal from working completely. This fix
includes both a 1.4 regression fix as well as a fix for a related issue
that existed in 1.3 also.   As part of this change, the
:class:`sqlalchemy.ext.instrumentation.InstrumentationManager` class now
has a new method ``unregister()``, which replaces the previous method
``dispose()``, which was not called as of version 1.4.

Fixes: #6390
Change-Id: I1b7a0f190b56d31b93b5ba11f67dc9f73889958b