I can't see any negative consequence of starting to listen to
postMessage discovery message immediately after `PortProvider` is
instantiated. (In fact, this is what it was currently happening). These
messages won't be lost, and the `PortRPC`s won't create the
communication channels before the RPC methods are registered.

On the other hand, it simplifies the use of `PortProvider` and
eliminates the potential problem of `PortProvider#listen` been called
more than once.