Improve KeyValueHeap using loser tree

LoserTree is the better data structure than binary heap. It saves half of the comparisons on each next(), though the time complexity is on O(logN).

Currently A scan or get will go through two KeyValueHeaps, one is merging KVs read from multiple HFiles in a single store, the other is merging results from multiple stores. This patch should improve the both cases whenever CPU is the bottleneck (e.g. scan with filter over cached blocks, HBASE-9811).

All of the optimization work is done in KeyValueHeap and does not change its public interfaces. The new code looks more cleaner and simpler to understand.