Cleans up mbskip encoding

Refactors mbskip coding to be compatible with coding of the rest of
the symbols. Adds forward/backward adaptation and removes a lot of
the legacy code.

Results:
fast50: +1.6%
derfraw300: +0.317%

Change-Id: I395a2976d15af044d3b8ded5acfa45f6f065f980