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 `CompactionJob`, and
`full_history_ts_low_` does not change during compaction. `CompactionJob` will pass a pointer to this
data member to the `CompactionIterator` used during compaction.

Also refactored compaction_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/7657

Test Plan: make check

Reviewed By: ltamasi

Differential Revision: D24913803

Pulled By: riversand963

fbshipit-source-id: 11ad5329ddac365667152e7b3b02f84182c0ca8e