Ensure all known BFT messages are imported when starting consensus (#147)

* a little more BFT tracing

* import cached BFT messages into the produced stream

substrate/substrate/bft/src/generic/accumulator.rs

@@ -339,6 +339,7 @@ impl<Candidate, Digest, AuthorityId, Signature> Accumulator<Candidate, Digest, A
 				count.committed += 1;
 
 				if count.committed >= self.threshold {
+					trace!(target: "bft", "observed threshold-commit for round {} with {} commits", self.round_number, count.committed);
 					Some(digest)
 				} else {
 					None

substrate/substrate/bft/src/lib.rs

@@ -348,6 +348,7 @@ impl<P, I> BftService<P, I>
 
 		let n = authorities.len();
 		let max_faulty = max_faulty_of(n);
+		trace!(target: "bft", "max_faulty_of({})={}", n, max_faulty);
 
 		let local_id = self.key.public().0;