Split header MMR (and sync MMR) out from txhashset (#3004)

* wip

* sync sort of works now

* get rid of the deadlock during compaction
there is *always* a deadlock in there when we make changes like this...

* cleanup how we rebuild the sync MMR on init

* cleanup rewind logic

* roll the "fix invalid root" changes into this PR

* move rebuild_height_pos_index into txhashset
and pass in header_pmmr for header lookups

* cleanup and remember to setup sync head on init

* cleanup unnecessary ref muts

* rebuild_height_pos_index when writing txhashset

core/src/core/pmmr/pmmr.rs

@@ -216,13 +216,6 @@ where
 		Ok(())
 	}
 
-	/// Truncate the MMR by rewinding back to empty state.
-	pub fn truncate(&mut self) -> Result<(), String> {
-		self.backend.rewind(0, &Bitmap::create())?;
-		self.last_pos = 0;
-		Ok(())
-	}
-
 	/// Rewind the PMMR to a previous position, as if all push operations after
 	/// that had been canceled. Expects a position in the PMMR to rewind and
 	/// bitmaps representing the positions added and removed that we want to

core/src/core/pmmr/readonly_pmmr.rs

@@ -86,6 +86,11 @@ where
 		}
 	}
 
+	/// Iterator over current (unpruned, unremoved) leaf positions.
+	pub fn leaf_pos_iter(&self) -> impl Iterator<Item = u64> + '_ {
+		self.backend.leaf_pos_iter()
+	}
+
 	/// Is the MMR empty?
 	pub fn is_empty(&self) -> bool {
 		self.last_pos == 0