fix bug where we over-eagerly remove backing spans for candidates we validate ourselves (#2142)

* fix bug where we over-eagerly remove backing spans for candidates we validate ourselves

* jaeger: watch importing of statements

polkadot/node/primitives/src/lib.rs

@@ -63,6 +63,17 @@ pub enum Statement {
 }
 
 impl Statement {
+	/// Get the candidate hash referenced by this statement.
+	///
+	/// If this is a `Statement::Seconded`, this does hash the candidate receipt, which may be expensive
+	/// for large candidates.
+	pub fn candidate_hash(&self) -> CandidateHash {
+		match *self {
+			Statement::Valid(ref h) | Statement::Invalid(ref h) => *h,
+			Statement::Seconded(ref c) => c.hash(),
+		}
+	}
+
 	/// Transform this statement into its compact version, which references only the hash
 	/// of the candidate.
 	pub fn to_compact(&self) -> CompactStatement {