Code a method to determine the activation block before any block has consensus

[0; 32] is a magic for no block has been set yet due to this being the first
key pair. If [0; 32] is the latest finalized block, the processor determines
an activation block based on timestamps.

This doesn't use an Option for ergonomic reasons.

processor/src/coins/bitcoin.rs

@@ -204,6 +204,10 @@ impl BlockTrait<Bitcoin> for Block {
     hash
   }
 
+  fn time(&self) -> u64 {
+    self.header.time.into()
+  }
+
   fn median_fee(&self) -> Fee {
     // TODO
     Fee(20)