config: provide settings for the LevelDB stores we use

Now that we can set up the LevelDB options internally, provide
config options on the OSD and the Monitor. We leave the OSD values
at the defaults for now as they're performance-sensitive, but we
set new values on the Monitor so that it can scale to large PGMaps.
(Previously there were issues with large PGMaps taking forever to write;
these changes to the use of compression and the default block and
write buffers counteract them.)

Since we pass these variables through, users who are interested in
doing so now can test and tune them more appropriately.

Reported-by: Jim Schutt <jaschut@sandia.gov>
Signed-off-by: Greg Farnum <greg@inktank.com>