Persistent predicates based on RocksDB

Request For Comments
91

@Rscho314

The first trial load of the SemMedDB data completed, took about 14 hours to load.

This line from the RocksDB log is notable.
Note: Split line up into multiple lines to improve readbility.

Cumulative writes: 
   339M writes, 
   339M keys, 
   339M commit groups,
   1.0 writes per commit group,
ingest:
   34.14 GB,
   0.70 MB/s