Summary:
When out of space, blob db should GC the oldest file. The current implementation GC the newest one instead. Fixing it.
Closes https://github.com/facebook/rocksdb/pull/2757
Differential Revision: D5657611
Pulled By: yiwu-arbug
fbshipit-source-id: 56c30a4c52e6ab04551dda8c5c46006d4070b28d