stress MonitorDBStore && have mon overriding leveldb's compression policy

add test/mon/mon-store-stress to stress MonitorDBStore

and

fix an annoying bug on MonitorDBStore::init_options() which was ignoring
mon_leveldb_compression and mon_leveldb_paranoid_checks if those
happened to be 'false'.  This helps the stress test a bit in terms of cpu usage,
but not significantly.  In larger clusters however it *may* be a big help.
This unfortunately means that the mon_leveldb_compression option always takes precedence over the leveldb_compression option.  Since the mon_leveldb_* configs are basically deprecated in favor of leveldb_*, maybe we should just drop these instead?

In fact, we might consider dropping all of mon_leveldb_* and osd_leveldb_* since we're post-firefly...