Limit RocksDB memory consumption

In the scope of INDY-1821 it was found that RockDB's readers-mem is increasing.

This looks like the main cause of slow memory increasing during long load tests.

Acceptance criteria

• Find out RocksDB config to limit readers-mem
• Find out if RocksDB wrapper keeps Python objects not-GCed
• [Optional] Get metrics about BlockCache (it may require changes in wrapper)
• [Optional] Explicitly set limits for BlockCache