Add a new primitive of a DB-backed channel

The coordinator already had one of these, albeit implemented much worse than
the one now properly introduced. It had to either be sending or receiving,
whereas the new one can do both at the same time.

This replaces said instance and enables pleasant patterns when implementing the
processor/coordinator.

coordinator/src/p2p.rs

@@ -37,7 +37,6 @@ pub(crate) use tributary::{ReadWrite, P2p as TributaryP2p};
 
 use crate::{Transaction, Block, Tributary, ActiveTributary, TributaryEvent};
 
-// TODO: Use distinct topics
 const LIBP2P_TOPIC: &str = "serai-coordinator";
 
 #[derive(Clone, Copy, PartialEq, Eq, Hash, Debug, Encode, Decode)]