Rate control changes on active_worst_quality

Various cleanups and refactoring.
Removes feedback of active worst qaulity and uses last_q
instead to make the interface cleaner. Active worst quality
is now decided only once for a frame being coded in the
beginning based on last_q and other stats. Also, adds other
cleaups on last_q to store also the last_q for altref frames,
and reduces the altref interval a little.

The output does change a little.
derfraw300: +0.224% (global psnr)
stdhdraw250: +0.442% (global psnr)

Change-Id: Ie634cdc032697044c472dd0fe79c109b3e7f9767