Summary:
Previously the WAL files that were avoided during recovery would never
be considered for deletion. That was because alive_log_files_ was only
populated when log files are created. This diff further populates
alive_log_files_ with existing log files that aren't flushed during recovery,
such that FindObsoleteFiles() can find them later.

Depends on D64053.

Test Plan: new unit test, verifies it fails before this change and passes after

Reviewers: sdong, IslamAbdelRahman, yiwu

Reviewed By: yiwu

Subscribers: leveldb, dhruba, andrewkr

Differential Revision: https://reviews.facebook.net/D64059