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/coins/bitcoin.rs

@@ -1,8 +1,4 @@
-use std::{
-  time::{SystemTime, Duration},
-  io,
-  collections::HashMap,
-};
+use std::{time::Duration, io, collections::HashMap};
 
 use async_trait::async_trait;
 
@@ -201,9 +197,6 @@ impl BlockTrait<Bitcoin> for Block {
     hash.reverse();
     hash
   }
-  fn time(&self) -> SystemTime {
-    SystemTime::UNIX_EPOCH + Duration::from_secs(self.header.time.into())
-  }
   fn median_fee(&self) -> Fee {
     // TODO
     Fee(20)