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