Handle adding new Tributaries

Removes last_block as an argument from Tendermint. It now loads from the DB as
needed. While slightly less performant, it's easiest and should be fine.

coordinator/src/tests/tributary/mod.rs

@@ -15,6 +15,7 @@ pub use chain::*;
 mod tx;
 
 mod dkg;
+// TODO: Test the other transactions
 
 fn random_u32<R: RngCore>(rng: &mut R) -> u32 {
   u32::try_from(rng.next_u64() >> 32).unwrap()