vmd_free_count manipulation.  Reduce the scope of the free lock by
using a pageout lock to synchronize sleep and wakeup.  Only trigger
the pageout daemon on transitions between states.  Drive all wakeup
operations directly as side-effects from freeing memory rather than
requiring an additional function call.

Reviewed by:	markj, kib
Tested by:	pho
Sponsored by:	Netflix, Dell/EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D14612