mgr/snap_schedule: fix db connection concurrent usage

mgr/snap_schedule: fix db connection concurrent usage

Serialize access to DB connection to avoid transaction aborts due to
concurrent use.

Some flake8-3.9 and mypy parsing error cleanups to keep 'make check' happy.

Fixes: https://tracker.ceph.com/issues/52642
Signed-off-by: Milind Changire <mchangir@redhat.com>
(cherry picked from commit 707543779e24c6bc1489c07f5fa1a239d110d9fb)

Conflicts:
	src/pybind/mgr/snap_schedule/fs/schedule.py
	src/pybind/mgr/snap_schedule/fs/schedule_client.py
        - changes related to DBConnectionManager to serialize
          db interactions