MonitorDBStore foo
mon: MonitorDBStore: force exists(str,v) to use our own exists(str,str)


Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
mon: MonitorDBStore: use get() on exists() instead of lower_bound()

This works around an issue with leveldb.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>