Merged https://github.com/facebook/mysql-5.6/commit/615dd07d900ce08948340b6c11e37fd1d2a91065 from https://github.com/facebook/mysql-5.6/
authored by rongrong. Removed C++11 requirement by using
std::map instead of std::unordered_set.

Add analysis to leaf pages to estimate how fragmented an index is
and how much benefit we can get out of defragmentation.