Correct processor flow to have the coordinator decide signing set/re-attempts

The signing set should be the first group to submit preprocesses to Tributary.
Re-attempts shouldn't be once every 30s, yet n blocks since the last relevant
message.

Removes the use of an async task/channel in the signer (and Substrate signer).
Also removes the need to be able to get the time from a coin's block, which was
a fragile system marked with a TODO already.

processor/src/tests/key_gen.rs

@@ -122,7 +122,7 @@ pub async fn test_key_gen<C: Coin>() {
     let key_gen = key_gens.get_mut(&i).unwrap();
     if let KeyGenEvent::KeyConfirmed { activation_block, substrate_keys, coin_keys } = key_gen
       .handle(CoordinatorMessage::ConfirmKeyPair {
-        context: SubstrateContext { time: 0, coin_latest_finalized_block: BlockHash([0x11; 32]) },
+        context: SubstrateContext { coin_latest_finalized_block: BlockHash([0x11; 32]) },
         id: ID,
       })
       .await