Summary:
This diff fixes a case when the forward iterator misses a new
insert when the mutable iterator is not current. The test is also
improved and the check for deleted iterators is made more informative.

Test Plan: DBTailingIteratorTest.*Trim

Reviewers: tnovak, sdong

Reviewed By: sdong

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D46167