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>