Summary:
The patch makes it possible to set the BlobDB configuration option
`garbage_collection_cutoff` on the command line. In addition, it changes
the `db_bench` code so that the default values of BlobDB related
parameters are taken from the defaults of the actual BlobDB
configuration options (note: this changes the the default of
`blob_db_bytes_per_sync`).
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6211
Test Plan: Ran `db_bench` with various values of the new parameter.
Differential Revision: D19166895
Pulled By: ltamasi
fbshipit-source-id: 305ccdf0123b9db032b744715810babdc3e3b7d5
130e710056