Import Facebook MySQL my_io_perf* functionality for MyRocks 5.6

MyRocks makes use of some new aggregated performance counters from Facebook MySQL 5.6.
We submitted a patch upstream that factors these out and C++-izes them a little which was accepted.
We now need to import the upstream include/atomic_stat.h and include/my_io_perf.h files into Percona Server include directory.
Alternately, we can pull these directly into MyRocks, but it seems they might be useful elsewhere, particularly if at some later point we decide to implement some of the better table statistics functionality from Facebook MySQL.