Previously, we had one callback for each kind of message that a `ConnectionHandler` would receive from either its `NetworkBehaviour` or the connection itself.
With this patch, we combine these functions, resulting in two callbacks:
- `on_behaviour_event`
- `on_connection_event`
Resolves#3080.
A `ProtocolsHandler`, now `ConnectionHandler`, handels a connection, not
a protocol. Thus the name `CONNECTIONHandler` is more appropriate.
Next to the rename of `ProtocolsHandler` this commit renames the `mod
protocols_handler` to `mod handler`. Finally all combinators (e.g.
`ProtocolsHandlerSelect`) are renamed appropriately.
