Remove MKF buffer or use a priority queue

[SabaAflaki]

./Tracker.h:638: std::vector< MultiKeyFrame* > mvKeyFrameBuffer; ///< vector that holds MKF pointers

We can either remove the buffer or use a priority queue (That keeps the N latest MKFs) instead.

[SabaAflaki]

Priority queue will be helpful only in the current version where we can infinitely add MKFs (no buffer size restrictions). With a limited buffer size to keep the last N MKFs, after the priority queue is full, every time we want to insert an MKF we have to remove the oldest MKF and insert the new one which is O(logn) as opposed to O(1) in a circular buffer. So, I'll go with Arun's opinion for a limited buffer size and a circular buffer.