Right now the API handler directly calls static method from system
keyspace. Patching it to call compaction manager instead will let the
latter use on-board plugged system keyspace for that. If the system
keyspace is not plugged, it means early boot or late shutdown, not a
good time to get compaction history anyway.
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>