The Header MMR (One MMR To Rule Them All) (#1716)

* header MMR in use within txhashset itself
works with fast sync
not yet in place for initial header sync

* add the (currently unused) sync_head mmr

* use sync MMR during fast sync
rebuild header MMR after we validate full txhashset after download

* support missing header MMR (rebuild as necessary) for legacy nodes

* rustfmt

* comments/docs

* rustfmt

* cleanup DBBackend

* cleanup DBBackend

* cleanup

* rename to HashOnly

* rustfmt

* cleanup backend.append()

* simply pmmr append api
no need to pass position when appending

* cleanup

* simplify vec_backend to match simpler append api

* rustfmt

* docs/comments

* rustfmt

* cleanup

api/src/types.rs

@@ -97,9 +97,9 @@ impl TxHashSet {
 	pub fn from_head(head: Arc<chain::Chain>) -> TxHashSet {
 		let roots = head.get_txhashset_roots();
 		TxHashSet {
-			output_root_hash: roots.0.to_hex(),
-			range_proof_root_hash: roots.1.to_hex(),
-			kernel_root_hash: roots.2.to_hex(),
+			output_root_hash: roots.output_root.to_hex(),
+			range_proof_root_hash: roots.rproof_root.to_hex(),
+			kernel_root_hash: roots.kernel_root.to_hex(),
 		}
 	}
 }