Re-model extra horizontal buffer to a counter for "lost sprint".

The problems with attacking + sprint/bunny seems to be that the player 
moves on as if sprinting, thus we reformulate it as the "lost sprint"
problem. The sprintback check is skipped during lostsprint phases.

This should be worth a ticket for Mojang/CB, sprint + jump + attack 
somewhere around touch down time (+-) - server side sprinting turns off,
but the player moves on as if sprinting.