Improve asyncpg exception hierarchy and asyncio hierarchies overall

Improve asyncpg exception hierarchy and asyncio hierarchies overall

The "emulated" exception hierarchies for the asyncio
drivers such as asyncpg, aiomysql, aioodbc, etc. have been standardized
on a common base :class:`.EmulatedDBAPIException`, which is now what's
available from the :attr:`.StatementException.orig` attribute on a
SQLAlchemy :class:`.DBAPIException` object.   Within :class:`.EmulatedDBAPIException`
and the subclasses in its hiearchy, the original driver-level exception is
also now avaliable via the :attr:`.EmulatedDBAPIException.orig` attribute,
and is also available from :class:`.DBAPIException` directly using the
:attr:`.DBAPIException.driver_exception` attribute.

Added additional emulated error classes for the subclasses of
``asyncpg.exception.IntegrityError`` including ``RestrictViolationError``,
``NotNullViolationError``, ``ForeignKeyViolationError``,
``UniqueViolationError`` ``CheckViolationError``,
``ExclusionViolationError``.  These exceptions are not directly thrown by
SQLAlchemy's asyncio emulation, however are available from the
newly added :attr:`.DBAPIException.driver_exception` attribute when a
:class:`.IntegrityError` is caught.

Fixes: #8047
Change-Id: I6a34e85b055265c087b0615f7c573be8582b3486