mon: handle bad snapshot removal reqs gracefully

mon: handle bad snapshot removal reqs gracefully

Snapshot deletion requests on snap ids larger than the snap_seq of
the pool will leave the pool in a state with snap_seq being less
than max(removed_snaps).

This is bad because further deletion requests to a pool in this state
might crash the mon in some cases: the deletion also inserts the new
snap_seq into the removed_snaps set -- which might already exist
in this case and trigger an assert.

Such bad requests will be generated by rbd clients without a fix for
issue https://tracker.ceph.com/issues/21567.

The change in OSDMonitor prevents pools from getting into this state
and may prevent old broken clients from incorrectly deleting snaps.
The change in osd_types avoids a crash if a pool is already in this
state.

Fixes: https://tracker.ceph.com/issues/18746
Signed-off-by: Paul Emmerich <paul.emmerich@croit.io>
(cherry picked from commit f42a6ba4c37cb10869718ec296352ae55254b44a)

Conflicts:
	src/osd/osd_types.cc: trivial resolution