crimson/osd/osd_operations/snaptrim_event: fix lifetime management

crimson/osd/osd_operations/snaptrim_event: fix lifetime management

The operation's id and future returned when starting SnapTrimObjSubEvent
is emplaced into subop_blocker.
Later on, we await the completion of all the started operations futures.
Before this patch, we only stored the op id in the subop_blocker vector
which allowed `op` to go out of scope and lose all its references
(and get deleted) before exiting.

Storing the operation as a reference instead of the id
will maintain the SnapTrimObjSubEvent operation lifetime.

Fixes: https://tracker.ceph.com/issues/63299
Signed-off-by: Matan Breizman <mbreizma@redhat.com>