compaction: simplify removal of monitors
compaction: simplify removal of monitors

by switching to unordered_map, removal of generated monitors is
made easier. this is a preparatory change for patch which will
remove monitor for all exhausted sstables

Signed-off-by: Raphael S. Carvalho <1e2942cea40e3b4d2162dfcd9e53e41c05a84d22@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla