mon: compact trimmed range, not entire prefix

mon: compact trimmed range, not entire prefix

This will reduce the work that leveldb is asked to do by only triggering
compaction of the keys that were just trimmed.

We ma want to further reduce the work by compacting less frequently, but
this is at least a step in that direction.

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 6da4b20ca53fc8161485c8a99a6b333e23ace30e)