wireprotov2: remove functions for creating response frames from bytes
All code in the actual server uses oncommandresponsereadyobjects().
Test code was ported to that method. This resulted in a handful of
subtle test changes.
Differential Revision: https://phab.mercurial-scm.org/D4924
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.