mon: DataHealthService: monitor backing store's size and report it

If the store's size grows beyond what we believe to be reasonable, we must
let the user know that something fishy may be going on.  This intends to
act as an early warning system for monitors suffering from leveldb
compaction issues.  However, if the monitor's store is just growing a lot
due to normal cluster behaviour, we made sure that the warning threshold
is adjustable by tuning 'mon_leveldb_size_warn' (defaulting to 40GB).

Fixes: #5909

Signed-off-by: Joao Eduardo Luis <jecluis@gmail.com>