invoke mariadb-connector .rowcount after all statements

invoke mariadb-connector .rowcount after all statements

Modified the mariadb-connector driver to pre-load the ``cursor.rowcount``
value for all queries, to suit tools such as Pandas that hardcode to
calling :attr:`.Result.rowcount` in this way. SQLAlchemy normally pre-loads
``cursor.rowcount`` only for UPDATE/DELETE statements and otherwise passes
through to the DBAPI where it can return -1 if no value is available.
However, mariadb-connector does not support invoking ``cursor.rowcount``
after the cursor itself is closed, raising an error instead.  Generic test
support has been added to ensure all backends support the allowing
:attr:`.Result.rowcount` to succceed (that is, returning an integer value
with -1 for "not available") after the result is closed.

This change also restores mariadb-connector to CI including
as part of the "dbdriver" suite; in 366a5e3e2e503a20ef0334fbf9f we had
taken it out of the DBAPI main job.

Additional fixes for the mariadb-connector dialect to support UUID data
values in the result in INSERT..RETURNING statements.

Added rounding to one remaining INSERT..RETURNING with floats test
to allow mariadbconnector to pass (likely similar issue as the one with
UUID but not worth making a new handler)

Fixes: #10396
Change-Id: Ic11b1b5d0c41356863829d0eacbb812d401e8dd1