Document more TODOs as tickets (#1418)

Went through the TODOs, removed a bunch, which are outdated or nothing more than a regular comment, documented a bunch more as actual tickets and made them FIXMEs and unified their structure (`FIXME #TICKETNO DESC` for local tickets, `FIXME: DESC LINK` for external tickets) for easier in-editor support. Further more remove unnecessary remarks and related old code that I noticed in that instance.

substrate/core/state-db/src/noncanonical.rs

@@ -220,7 +220,7 @@ impl<BlockHash: Hash, Key: Hash> NonCanonicalOverlay<BlockHash, Key> {
 				commit.data.inserted = self.last_canonicalized_overlay.iter().map(|(k, v)| (k.clone(), v.clone())).collect();
 				commit.data.deleted = overlay.deleted;
 			} else {
-				// TODO: borrow checker won't allow us to split out mutable references
+				// borrow checker won't allow us to split out mutable references
 				// required for recursive processing. A more efficient implementation
 				// that does not require converting to vector is possible
 				let mut vec: Vec<_> = self.levels.drain(..).collect();