Implement "cephfs-journal-tool event splice" equivalent for purge queue
cephfs-journal-tool recently got the ability to scan the purge queue via the --journal=purge_queue argument.
However, all the 'event' mode commands are disable for purge queue, because it relies on the actual contained event format (e.g. ENoOp for splice).
When the purge queue scan reports a region of damage, we need a command for splicing out that region.