- BUG#22306: STOP INSTANCE can not be applied for instances in Crashed,
    Failed and Abandoned;
  - BUG#23476: DROP INSTANCE does not work
  - BUG#23215: STOP INSTANCE takes too much time

BUG#22306:
The problem was that STOP INSTANCE checked that mysqld is up and running.
If it was not so, STOP INSTANCE reported an error. Now, STOP INSTANCE
reports an error if the instance has been started (mysqld can be down).

BUG#23476:
The problem was that DROP INSTANCE tried to stop inactive instance. The fix is
trivial.

BUG#23215:
The problem was that locks were not acquired properly, so the
instance-monitoring thread could not acquire the mutex, holded by the
query-processing thread.

The fix is to simplify locking scheme by moving instance-related information to
Instance-class out of Guardian-class. This allows to get rid of storing a
separate list of Instance-information in Guardian and keeping it synchronized
with the original list in Instance_map.