Merge Facebook commit f981a51a47519b0ba527917887f8adc6df9ae147
authored by Steaphan Greene from https://github.com/facebook/mysql-5.6.

This just moves some structure definitions from inside a
single .cc file to a shared .h file, with a few tweaks to
allow these structures to be shared.

On its own, it should have no actual effect. This is needed later.