Add a networking message to enable/disable the Substrate protocol on connections

[tomaka]

At the moment, we assume that a connection can be in two states: open and talking the Substrate protocol, or closed.
If a node is full, is in reserved mode, or if the peer is banned, we simply close the connection in order to stop talking the Substrate protocol.

There's a potential third state: performing a Kademlia request, or in the future some off-chain pure-libp2p communication, where we don't have to talk the Substrate protocol.

We can already keep connections alive and deny request for the Substrate protocol; however we have no way to then indicate that we will now start accepting the Substrate protocol if we denied it earlier.
We therefore need to send some kind of message between the two nodes in order to indicate that.

[tomaka]

Closing this as it is covered by #5670