os: bring leveldbstore options up to date

LevelDB has a lot of options which we don't implement right now. Add
an options struct to the LevelDBStore which users can access as they
wish in order to set values different from the defaults.
This will let us set various size values, as well as turning on
caching or bloom filter read optimizations.

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