Add basic support for user-defined timestamp to db_bench (#7389)

Summary:
Update db_bench so that we can run it with user-defined timestamp.
Currently, only 64-bit timestamp is supported, while others are disabled by assertion.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7389

Test Plan: ./db_bench -benchmarks=fillseq,fillrandom,readrandom,readsequential,....., -user_timestamp_size=8

Reviewed By: ltamasi

Differential Revision: D23720830

Pulled By: riversand963

fbshipit-source-id: 486eacbb82de9a5441e79a61bfa9beef6581608a