Use RocksDB default settings

Before and after:

NumObjects: 10000
  Batch Insert    Fetch 50/50  Fetch Missing   Fetch Random
Inserts  Ordered Fetch
          0.34           0.14           0.15           0.15
0.64           0.13 type=rocksdb
          0.11           0.18           0.16           0.25
0.19           0.16 type=rocksdb

NumObjects: 100000
  Batch Insert    Fetch 50/50  Fetch Missing   Fetch Random
Inserts  Ordered Fetch
          21.65          16.69          12.33          15.24
20.75          10.27 type=rocksdb
          3.69           3.45           3.97           4.02
2.04           3.58 type=rocksdb

NumObjects: 1000000
  Batch Insert    Fetch 50/50  Fetch Missing   Fetch Random
Inserts  Ordered Fetch
        480.86         137.44          43.95          98.75
582.94         123.21 type=rocksdb
        273.38          56.10          72.14          47.82
116.59          52.06 type=rocksdb