Add a TributaryReader which doesn't require a borrow to operate

Reduces lock contention.

Additionally changes block_key to include the genesis. While not technically
needed, the lack of genesis introduced a side effect where any Tributary on the
the database could return the block of any other Tributary. While that wasn't a
security issue, returning it suggested it was on-chain when it wasn't. This may
have been usable to create issues.

coordinator/tributary/src/tests/blockchain.rs

@@ -43,7 +43,7 @@ fn block_addition() {
   assert_eq!(blockchain.tip(), block.hash());
   assert_eq!(blockchain.block_number(), 1);
   assert_eq!(
-    Blockchain::<MemDb, SignedTransaction>::block_after(&db, &block.parent()).unwrap(),
+    Blockchain::<MemDb, SignedTransaction>::block_after(&db, genesis, &block.parent()).unwrap(),
     block.hash()
   );
 }