[RocksDB] Fix a race condition in GetSortedWalFiles

Summary: This patch fixed a race condition where a log file is moved to archived dir in the middle of GetSortedWalFiles. Without the fix, the log file would be missed in the result, which leads to transaction log iterator gap. A test utility SyncPoint is added to help reproducing the race condition.

Test Plan: TransactionLogIteratorRace; make check

Reviewers: dhruba, ljin

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17121