Summary:
Currently, db_stress generates fixed length keys of 8 bytes. This patch adds the ability to generate variable length keys. Most of the db_stress code continues to work with a numeric key randomly generated, and the numeric key also acts as an index into the values_ array. The numeric key is mapped to a variable length string key in a deterministic way. Furthermore, the ordering is preserved.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6165

Test Plan: run make crash_test

Differential Revision: D19204646

Pulled By: anand1976

fbshipit-source-id: d2d46a96615b4832a8be2a981f5913905f0e1ca7