Summary:
This change has the crash test randomly select from a few file
checksum implementations, or nullptr, for DB file_checksum_gen_factory.
For compatibility across runs on same DB, each non-null factory can
understand all the other functions, but the default changes.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7343

Test Plan:
'make blackbox_crash_test' for a while, including with some
debug output to ensure code is being exercised.

Reviewed By: zhichao-cao

Differential Revision: D23494580

Pulled By: pdillinger

fbshipit-source-id: 73bbc7ca32c1adaf619134c0c830f12894880b8a