Summary:
While doing forward iterating, if current key is merge, internal iterator position is placed to the next key. If Prev() is called now, needs to do extra Prev() to recover the location.
This is second attempt of fixing after reverting ec70fea4. This time shrink the fix to only merge key is the current key and avoid the reseeking logic for max_iterating skipping

Test Plan: enable the two disabled tests and make sure they pass

Reviewers: rven, IslamAbdelRahman, kradhakrishnan, tnovak, yhchiang

Reviewed By: yhchiang

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D43557