mon: Encapuslate all scrub related objects into a single atomic ScrubContext

mon: Encapuslate all scrub related objects into a single atomic ScrubContext

During handle of scrub operation if scrub_state is reset via
bootstrap then a process might be crash.

Solution: Encapsulate all scrub related objects into a single
          ScrubContext and manage it as via boost::atomic_shared_ptr.

Fixes: https://tracker.ceph.com/issues/67270
Credits: Radoslaw Zarzynski <rzarzyns@redhat.com>
Signed-off-by: Mohit Agrawal <moagrawa@redhat.com>