The Header MMR (One MMR To Rule Them All) (#1716) (#1747)

* 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

* rename to HashOnly

* cleanup backend.append()

* simplify vec_backend to match simpler append api

servers/src/grin/sync/header_sync.rs

@@ -67,7 +67,13 @@ impl HeaderSync {
 					header_head.hash(),
 					header_head.height,
 				);
+
+				// Reset sync_head to the same as current header_head.
 				self.chain.reset_sync_head(&header_head).unwrap();
+
+				// Rebuild the sync MMR to match our updates sync_head.
+				self.chain.rebuild_sync_mmr(&header_head).unwrap();
+
 				self.history_locators.clear();
 				true
 			}