qa/tasks: make rbd_mirror_thrash inherit from ThrasherGreenlet

qa/tasks: make rbd_mirror_thrash inherit from ThrasherGreenlet

Commit 21b4b89e5280 ("qa/tasks: watchdog terminate thrasher") made it
required for a thrasher to have stop_and_join() method, but the
preceding commit a035b5a22fb8 ("thrashers: standardize stop and join
method names") missed to add it to rbd_mirror_thrash (whether as an
ad-hoc implementation or by way of inheriting from ThrasherGreenlet).
Later on, commit 783f0e3a9903 ("qa: Adding a new class for the
daemonwatchdog to monitor") worsened the issue by expanding the use
of stop_and_join() to all watchdog barks rather than just the case of
a thrasher throwing an exception which is something that practically
never happens.

Fixes: https://tracker.ceph.com/issues/75200
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>