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Went through the TODOs, removed a bunch, which are outdated or nothing more than a regular comment, documented a bunch more as actual tickets and made them FIXMEs and unified their structure (`FIXME #TICKETNO DESC` for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-machine/src/proving_backend.rs

@@ -195,7 +195,7 @@ where
 	H: Hasher,
 	H::Out: HeapSizeOf,
 {
-	let mut db = MemoryDB::default();	// TODO: use new for correctness
+	let mut db = MemoryDB::default();
 	for item in proof {
 		db.insert(&item);
 	}