Draft: topic: simply reuse wirepeer.branchmap() as wirepeer.branchmaptns()

The function is just parsing bytes into a dict. In this way, branchmap and branchmaptns are completely compatible.

The only downside is that wirepeer.branchmaptns doesn't get proper function name, so if someone needs to debug this, they might still see "wirepeer.branchmap" in certain places even if we're calling branchmaptns.