Summary:
This is to avoid run time error. Fail the db_bench immediately if cuckoo table is used but mmap_read is not specified.
Closes https://github.com/facebook/rocksdb/pull/3420
Differential Revision: D6838284
Pulled By: siying
fbshipit-source-id: 20893fa28d40fadc31e4ff154bed02f5a1bad341