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/tributary/db.rs

@@ -9,6 +9,8 @@ use frost::Participant;
 
 use serai_client::validator_sets::primitives::{ValidatorSet, KeyPair};
 
+use processor_messages::coordinator::SubstrateSignableId;
+
 pub use serai_db::*;
 
 use crate::tributary::TributarySpec;
@@ -16,16 +18,21 @@ use crate::tributary::TributarySpec;
 #[derive(Clone, Copy, PartialEq, Eq, Debug)]
 pub enum Topic {
   Dkg,
-  Batch([u8; 5]),
+  SubstrateSign(SubstrateSignableId),
   Sign([u8; 32]),
 }
 
 impl Topic {
   fn as_key(&self, genesis: [u8; 32]) -> Vec<u8> {
     let mut res = genesis.to_vec();
+    #[allow(unused_assignments)] // False positive
+    let mut id_buf = vec![];
     let (label, id) = match self {
       Topic::Dkg => (b"dkg".as_slice(), [].as_slice()),
-      Topic::Batch(id) => (b"batch".as_slice(), id.as_slice()),
+      Topic::SubstrateSign(id) => {
+        id_buf = id.encode();
+        (b"substrate_sign".as_slice(), id_buf.as_slice())
+      }
       Topic::Sign(id) => (b"sign".as_slice(), id.as_slice()),
     };
     res.push(u8::try_from(label.len()).unwrap());