Summary: With recent commit 33e0c938, db iterator skips perf context counter internal_key_skipped_count when blindly issuing internal Next(). Now increment the counter by one when issuing this Next()

Test Plan: Run all existing tests

Reviewers: rven, yhchiang, IslamAbdelRahman, kradhakrishnan, igor, anthony

Reviewed By: anthony

Subscribers: yoshinorim, leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D51465