Allow registering multiple versions of a single notifications protocol

[tomaka]

cc #1518

The register_notifications_protocol method allows you to register a protocol, but there's no real way at the moment to "upgrade" the protocol to a new version.
One could register a second protocol with the new version, but that means you'd open both the old one and the new one with every single peer.

Instead, one should be able to pass multiple protocol names, and, on connection, which one was negotiated is indicated.