Backport Performance Schema Memory Instrumentation from MySQL 5.7

Since 5.7 MySQL Performance Schema include memory instrumentation that helps to understand or tune the memory consumption of the server.

Many memory-allocation related problems are difficult to investigate due to the lack of instrumentation. Currently, using gdb is the most straightforward way to examine memory usage, and understanding everything in THD is not easy. The 377 instruments provided by MySLQ 5.7 would contribute to better-understood memory consumption and so to more stable & performant servers.