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

Summary: Closes https://github.com/facebook/rocksdb/pull/2589

Differential Revision: D5431502

Pulled By: siying

fbshipit-source-id: 8ebf8c87883daa9daa54b2303d11ce01ab1f6f75

Summary:
Blob db use StackableDB::get which only get out the
value offset, but not the value.
Fix by making BlobDB::Get override the designated getter.
Closes https://github.com/facebook/rocksdb/pull/2553

Differential Revision: D5396823

Pulled By: yiwu-arbug

fbshipit-source-id: 5a7d1cf77ee44490f836a6537225955382296878

Summary:
`CompressBlock()` will return the uncompressed slice (i.e. `Slice(value_unc)`) if compression ratio is not good enough. This is undesired. We need to always assign the compressed slice to `value`.
Closes https://github.com/facebook/rocksdb/pull/2447

Differential Revision: D5244682

Pulled By: yiwu-arbug

fbshipit-source-id: 6989dd8852c9622822ba9acec9beea02007dff09

Summary:
I'm trying to improve unit test of blob db. I'm rewriting blob db test. In this patch:
* Rewrite tests of basic put/write/delete operations.
* Add disable_background_tasks to BlobDBOptionsImpl to allow me not running any background job for basic unit tests.
* Move DestroyBlobDB out from BlobDBImpl to be a standalone function.
* Remove all garbage collection related tests. Will rewrite them in following patch.
* Disabled compression test since it is failing. Will fix in a followup patch.
Closes https://github.com/facebook/rocksdb/pull/2446

Differential Revision: D5243306

Pulled By: yiwu-arbug

fbshipit-source-id: 157c71ad3b699307cb88baa3830e9b6e74f8e939

Summary:
Blob db rely on base db returning sequence number through write batch after DB::Write(). However after recent changes to the write path, DB::Writ()e no longer return sequence number in some cases. Fixing it by have WriteBatchInternal::InsertInto() always encode sequence number into write batch.

Stacking on #2375.
Closes https://github.com/facebook/rocksdb/pull/2385

Differential Revision: D5148358

Pulled By: yiwu-arbug

fbshipit-source-id: 8bda0aa07b9334ed03ed381548b39d167dc20c33

Summary:
The final pull request for Blob Storage.
Closes https://github.com/facebook/rocksdb/pull/2269

Differential Revision: D5033189

Pulled By: yiwu-arbug

fbshipit-source-id: 6356b683ccd58cbf38a1dc55e2ea400feecd5d06