Summary:
https://github.com/facebook/rocksdb/issues/7556 enables `CompactionIterator` to perform garbage collection during compaction according
to a lower bound (user-defined) timestamp `full_history_ts_low_`.
This PR adds a data member `full_history_ts_low_` of type `std::string` to `FlushJob`, and
`full_history_ts_low_` does not change during flush. `FlushJob` will pass a pointer to this data member
to the `CompactionIterator` used during flush.
Also refactored flush_job_test.cc to re-use some existing code, which is actually the majority of this PR.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7655
Test Plan: make check
Reviewed By: ltamasi
Differential Revision: D24933340
Pulled By: riversand963
fbshipit-source-id: 2e584bfd0cf6e5c295ab1af264e68e9d6a12fca3
76ef894f9f