Split AGI manager events, to remove SubEvent field.

This moves them to stasis, in the process.

(closes issue ASTERISK-21470)
Review: https://reviewboard.asterisk.org/r/2587/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@390701 65c4cc65-6c06-0410-ace0-fbb531ad65f3