This fixes support for proxying after last commit broke it.
Note that withP2PConnections is called at server startup, and so only
proxies seen at that point will appear in the map and be used. It was
already the case that a proxy added after p2phttp was running would not
be served.
I think that is possibly a bug, but at least this commit doesn't
introduce the problem, though it might make it harder to fix it.
As bugs go, it's probably not a big deal, because after all,
git configs needs to be set in the local repository, followed by
git-annex updateproxy being run, to set up proxying. If someone is doing
that, they can restart their http server I suppose.
07026cf58b