Summary:
Introducing blob_db::TTLExtractor to replace extract_ttl_fn. The TTL
extractor can be use to extract TTL from keys insert with Put or
WriteBatch. Change over existing extract_ttl_fn are:
* If value is changed, it will be return via std::string* (rather than Slice*). With Slice* the new value has to be part of the existing value. With std::string* the limitation is removed.
* It can optionally return TTL or expiration.

Other changes in this PR:
* replace `std::chrono::system_clock` with `Env::NowMicros` so that I can mock time in tests.
* add several TTL tests.
* other minor naming change.
Closes https://github.com/facebook/rocksdb/pull/2659

Differential Revision: D5512627

Pulled By: yiwu-arbug

fbshipit-source-id: 0dfcb00d74d060b8534c6130c808e4d5d0a54440