Summary:
Make blob_dump tool able to show uncompressed values if the blob file is compressed. Also show total compressed vs. raw size at the end if --show_summary is provided.
Closes https://github.com/facebook/rocksdb/pull/3633

Differential Revision: D7348926

Pulled By: yiwu-arbug

fbshipit-source-id: ca709cb4ed5cf6a550ff2987df8033df81516f8e