Make LevelDB operating modes invocation-time conf.

Presently our use of LevelDB and its operating modes are hardcoded
into the storage stack.  This pull request decouples this and
re-exposes this through flags.  We can now perform benchmarking
and remedial tuning.
Merge pull request #52 from prometheus/feature/leveldb/pluggable-defaults

Make LevelDB operating modes invocation-time conf.