Summary: Getting file size from all the backup files can take a long time. In some cases, the sizes are available in file names. We allow a mode to get those sizes from file name.
Test Plan:
Make some unit tests in backupable_db_test to run in such a mode.
Make sure RocksDB Lite builds too.
Reviewers: IslamAbdelRahman, rven, yhchiang, kradhakrishnan, anthony, igor
Reviewed By: igor
Subscribers: muthu, asameet, leveldb, dhruba
Differential Revision: https://reviews.facebook.net/D51243
