* kvdb-rocksdb: update to RocksDB 5.8.8

* kvdb-rocksdb: tune RocksDB options

* Switch to level-style compaction
* Increase default block size (16K), and use bigger blocks for HDDs (64K)
* Increase default file size base (64MB SSDs, 256MB HDDs)
* Create a single block cache shared across all column families
* Tune compaction settings using RocksDB helper functions, taking into account
  memory budget spread across all columns
* Configure backgrounds jobs based on the number of CPUs
* Set some default recommended settings

* ethcore: remove unused config blockchain.db_cache_size

* parity: increase default value for db_cache_size

* kvdb-rocksdb: enable compression on all levels

* kvdb-rocksdb: set global db_write_bufer_size

* kvdb-rocksdb: reduce db_write_bufer_size to force earlier flushing

* kvdb-rocksdb: use master branch for rust-rocksdb dependency