Revert previous behavior of internal_key_skipped_count

Summary: With recent commit 33e0c93826, 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

db/db_iter.cc

@@ -193,6 +193,7 @@ void DBIter::Next() {
     // If the current key is a merge, very likely iter already points
     // to the next internal position.
     iter_->Next();
+    PERF_COUNTER_ADD(internal_key_skipped_count, 1);
   }
 
   // Now we point to the next internal position, for both of merge and