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/src/tests/tributary/tx.rs

@@ -46,7 +46,7 @@ async fn tx_test() {
 
   // All tributaries should have acknowledged this transaction in a block
   for (_, tributary) in tributaries {
-    let block = tributary.block(&included_in).unwrap();
+    let block = tributary.reader().block(&included_in).unwrap();
     assert_eq!(block.transactions, vec![tx.clone()]);
   }
 }