Fix GetSortedWalFiles when log recycling enabled

Fix GetSortedWalFiles when log recycling enabled

Summary:
Previously the sequence number was mistakenly passed in an argument
where the log number should go. This caused the reader to assume the old WAL
format was used, which is incompatible with the WAL recycling format.

Test Plan:
new unit test, verified it fails before this change and passes
afterwards.

Reviewers: yiwu, lightmark, IslamAbdelRahman, sdong

Reviewed By: sdong

Subscribers: andrewkr, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D63987