Summary:
This fixes#3130525. Dhruba's suggestion and Tnovak's implementation :)
The issue was with SkipEmptyDataBlocksForward(), but I also changed SkipEmptyDataBlocksBackward(). Is that OK?
Test Plan: Run the logdevice test
Reviewers: dhruba
Reviewed By: dhruba
CC: leveldb
Differential Revision: https://reviews.facebook.net/D13911