Revert "clean up disable-polling setup (for tests)"

This reverts commit e8f21cf05ab5b135fa120326ce1b4737bad5733c.
Revert "Deferred-ify rendezvous sending and test_invitation."

This reverts commit 5994ba957589ab32320142fd061519af78b510e2.
add util.allocate_port()
allocate webport at node-create, not at runtime

This simplifies a lot of code, as the baseurl is known before the Node
is even created. The create-node CLI arguments were changed.

Also, webapi tokens are cleared earlier, in Node.init_webport(), since
the previous sequence wasn't avoiding the race condition mentioned in
the comments (which wasn't a real race anyways, since the reactor never
gets control at a time that would let the uncleared state be
observable).