mon: Early warning system for monitor stores growing over predefined threshold

Warn the user if the store is growing way more than we believe it should.  This can be used to detect monitors failing to compact the leveldb store, or even something fishy in the cluster that may not be allowing maps to be trimmed.  In any case, raise a WARN.

Said threshold is tunable via 'mon_leveldb_size_warn', defaulting to 40 GB.