Upgrade LevelDB to 1.12.0.

A number of core fixes have occurred in upstream LevelDB.  Given API
compatibility, we should upgrade.
Shrink default LRU cache sizes.

Observing Prometheus in production confirms we can lower these values
safely.
Disable paranoid checks and expose max FDs option.

We shouldn't need paranoid checks now.  We also shouldn't need
too many FDs being open due to rule evaluator hitting in-memory
values stream.
Merge pull request #313 from prometheus/feature/memory-profile/more-reasonable

Adjust LevelDB Defaults