Add a cosigning protocol to ensure finalizations are unique (#433)

* Add a function to deterministically decide which Serai blocks should be co-signed

Has a 5 minute latency between co-signs, also used as the maximal latency
before a co-sign is started.

* Get all active tributaries we're in at a specific block

* Add and route CosignSubstrateBlock, a new provided TX

* Split queued cosigns per network

* Rename BatchSignId to SubstrateSignId

* Add SubstrateSignableId, a meta-type for either Batch or Block, and modularize around it

* Handle the CosignSubstrateBlock provided TX

* Revert substrate_signer.rs to develop (and patch to still work)

Due to SubstrateSigner moving when the prior multisig closes, yet cosigning
occurring with the most recent key, a single SubstrateSigner can be reused.
We could manage multiple SubstrateSigners, yet considering the much lower
specifications for cosigning, I'd rather treat it distinctly.

* Route cosigning through the processor

* Add note to rename SubstrateSigner post-PR

I don't want to do so now in order to preserve the diff's clarity.

* Implement cosign evaluation into the coordinator

* Get tests to compile

* Bug fixes, mark blocks without cosigners available as cosigned

* Correct the ID Batch preprocesses are saved under, add log statements

* Create a dedicated function to handle cosigns

* Correct the flow around Batch verification/queueing

Verifying `Batch`s could stall when a `Batch` was signed before its
predecessors/before the block it's contained in was cosigned (the latter being
inevitable as we can't sign a block containing a signed batch before signing
the batch).

Now, Batch verification happens on a distinct async task in order to not block
the handling of processor messages. This task is the sole caller of verify in
order to ensure last_verified_batch isn't unexpectedly mutated.

When the processor message handler needs to access it, or needs to queue a
Batch, it associates the DB TXN with a lock preventing the other task from
doing so.

This lock, as currently implemented, is a poor and inefficient design. It
should be modified to the pattern used for cosign management. Additionally, a
new primitive of a DB-backed channel may be immensely valuable.

Fixes a standing potential deadlock and a deadlock introduced with the
cosigning protocol.

* Working full-stack tests

After the last commit, this only required extending a timeout.

* Replace "co-sign" with "cosign" to make finding text easier

* Update the coordinator tests to support cosigning

* Inline prior_batch calculation to prevent panic on rotation

Noticed when doing a final review of the branch.

coordinator/src/tests/tributary/sync.rs

@@ -5,7 +5,10 @@ use rand_core::OsRng;
 
 use ciphersuite::{group::GroupEncoding, Ciphersuite, Ristretto};
 
-use tokio::{sync::broadcast, time::sleep};
+use tokio::{
+  sync::{mpsc, broadcast},
+  time::sleep,
+};
 
 use serai_db::MemDb;
 
@@ -42,7 +45,8 @@ async fn sync_test() {
     let tributary = Arc::new(tributary);
     tributary_arcs.push(tributary.clone());
     let (new_tributary_send, new_tributary_recv) = broadcast::channel(5);
-    let thread = tokio::spawn(handle_p2p_task(p2p, new_tributary_recv));
+    let (cosign_send, _) = mpsc::unbounded_channel();
+    let thread = tokio::spawn(handle_p2p_task(p2p, cosign_send, new_tributary_recv));
     new_tributary_send
       .send(TributaryEvent::NewTributary(ActiveTributary { spec: spec.clone(), tributary }))
       .map_err(|_| "failed to send ActiveTributary")
@@ -77,7 +81,8 @@ async fn sync_test() {
   let syncer_key = Ristretto::generator() * *syncer_key;
   let syncer_tributary = Arc::new(syncer_tributary);
   let (syncer_tributary_send, syncer_tributary_recv) = broadcast::channel(5);
-  tokio::spawn(handle_p2p_task(syncer_p2p.clone(), syncer_tributary_recv));
+  let (cosign_send, _) = mpsc::unbounded_channel();
+  tokio::spawn(handle_p2p_task(syncer_p2p.clone(), cosign_send, syncer_tributary_recv));
   syncer_tributary_send
     .send(TributaryEvent::NewTributary(ActiveTributary {
       spec: spec.clone(),