Summary:
Add two unit tests for SyncWAL(). One makes sure SyncWAL() doesn't block writes in the other thread. Another one makes sure SyncWAL() doesn't wait ongoing writes to finish before being executed.

Create a new test file db_wal_test and move two WAL related tests from db_test to here.

Test Plan: Run the new tests

Reviewers: IslamAbdelRahman, rven, kradhakrishnan, kolmike, tnovak, yhchiang

Reviewed By: yhchiang

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D43605