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