First pass overhaul preparatory change.

This is the first patch in a series of changes to the first
pass code. (Broken down for ease of testing/merging/review).

This patch introduces a new stats element "sr_coded_error".
This is the coded error recorded vs the second reference
frame (which is updated such that it lags by at least one frame).
No use is made of the new structure in this change so this patch
should have no material effect.

Removed some ifdefs and deprecated code (#if NEW_BOOST).
Removed twopass.gf_decay_rate (not used any more)

Change-Id: I1be672a73017f7c13fd50fb4f99236aa2ed30916