Summary:
The patch adds logic that relocates live blobs from the oldest N non-TTL
blob files as they are encountered during compaction (assuming the BlobDB
configuration option `enable_garbage_collection` is `true`), where N is defined
as the number of immutable non-TTL blob files multiplied by the value of
a new BlobDB configuration option called `garbage_collection_cutoff`.
(The default value of this parameter is 0.25, that is, by default the valid blobs
residing in the oldest 25% of immutable non-TTL blob files are relocated.)
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6121

Test Plan: Added unit test and tested using the BlobDB mode of `db_bench`.

Differential Revision: D18785357

Pulled By: ltamasi

fbshipit-source-id: 8c21c512a18fba777ec28765c88682bb1a5e694e