(bug fix)  clear the register before cache clear.

Seems that all those "clear cache" problems were all related to stuck messages in the register.

Depends on: 
https://github.com/modxcms/revolution/pull/565
https://github.com/modxcms/revolution/pull/566

Solves long standing bugs on this:
http://tracker.modx.com/issues/413
http://tracker.modx.com/issues/3090
http://tracker.modx.com/issues/919
Solves an unreported bug:     Clear cache always show the last run's report,  not the current.