Summary:
Use a std::priority_queue in merger.cc instead of doing a o(n) search
every time.
Currently only the ForwardIteration uses a Priority Queue.
Test Plan: make all check
Reviewers: dhruba
Reviewed By: dhruba
CC: emayanke, zshao
Differential Revision: https://reviews.facebook.net/D7629
Abhishek Kona