Create fillanddeleteuniquerandom benchmark (db_bench), with new option flags. (#8593)

Create fillanddeleteuniquerandom benchmark (db_bench), with new option flags. (#8593)

Summary:
Introduction of a new `fillanddeleteuniquerandom` benchmark (`db_bench`) with 5 new option flags to simulate a benchmark where the following sequence is repeated multiple times:
"A set of keys S1 is inserted ('`disposable entries`'), then after some delay another set of keys S2 is inserted ('`persistent entries`') and the first set of keys S1 is deleted. S2 artificially represents the insertion of hypothetical results from some undefined computation done on the first set of keys S1. The next sequence can start as soon as the last disposable entry in the set S1 of this sequence is inserted, if the `delay` is non negligible."
New flags:
- `disposable_entries_delete_delay`: minimum delay in microseconds between insertion of the last `disposable` entry, and the start of the insertion of the first `persistent` entry.
- `disposable_entries_batch_size`: number of `disposable` entries inserted at the beginning of each sequence.
- `disposable_entries_value_size`: size of the random `value` string for the `disposable` entries.
- `persistent_entries_batch_size`: number of `persistent` entries inserted at the end of each sequence, right before the deletion of the `disposable` entries starts.
- `persistent_entries_value_size`: size of the random value string for the `persistent` entries.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8593

Reviewed By: pdillinger

Differential Revision: D29974436

Pulled By: bjlemaire

fbshipit-source-id: f578033e5b45e8268ba6fa6f38f4770c2e6e801d