Summary:
With FIFO compaction we would like to get the oldest data time for monitoring. The problem is we don't have timestamp for each key in the DB. As an approximation, we expose the earliest of sst file "creation_time" property.

My plan is to override the property with a more accurate value with blob db, where we actually have timestamp.
Closes https://github.com/facebook/rocksdb/pull/2842

Differential Revision: D5770600

Pulled By: yiwu-arbug

fbshipit-source-id: 03833c8f10bbfbee62f8ea5c0d03c0cafb5d853a