Determine ideal defaults for MyRocks/RocksDB options

default_cf_options - change from nullptr and set rational column family option defaults, specifically default compression plan.

rocksdb_max_open_files - must be lowered from infinite to sensible amount like 1000. Possibly do some auto-tune like InnoDB does if == 0 as well as limit to be no larger than open_tables_limit.

rocksdb_max_row_locks - should be lowered to limit memory used per transaction until https://github.com/facebook/mysql-5.6/issues/692 is fixed.

rocksdb_write_batch_max_bytes - When pulled in as part of MYR-47 should be lowered to limit memory used per transaction until https://github.com/facebook/mysql-5.6/issues/692 is fixed.

rocksdb_block_size - Needs rational lower limit in bytes, probably 4096 like TokuDB, see MYR-190