streamclone: move wire protocol status code from wireproto command
This consolidates the code for the streaming clone wire protocol format
into streamclone.py. It also eliminates a generator wrapper, which might
make streaming clones slightly faster.
Differential Revision: https://phab.mercurial-scm.org/D1754
#require docutils gettext
$ $TESTDIR/check-gendoc it
checking for parse errors