mon/MonitorDBStore: allow compaction of ranges

mon/MonitorDBStore: allow compaction of ranges

Allow a transaction to describe the compaction of a range of keys.  Do this
in a backward compatible say, such that older code will interpret the
compaction of a prefix + range as compaction of the entire prefix.  This
allows us to avoid introducing any new feature bits.

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit ab09f1e5c1305a64482ebbb5a6156a0bb12a63a4)

Conflicts:

	src/mon/MonitorDBStore.h