Benchmark + tune (if needed) rocksdb usage in Nimbus

[kdeme]

There are quite some settings tunable in rocksdb, see: https://github.com/facebook/rocksdb/wiki/RocksDB-Tuning-Guide

Of which we are recently changing one, see: https://github.com/status-im/nim-eth/pull/85

It would be good to benchmark and investigate the impact of some of these settings.

Something similar was done for Parity: https://github.com/paritytech/parity-ethereum/commit/e114b0b28d33e89342c4077da74d944fa175ebda