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Went through the TODOs, removed a bunch, which are outdated or nothing more than a regular comment, documented a bunch more as actual tickets and made them FIXMEs and unified their structure (`FIXME #TICKETNO DESC` for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

@@ -104,7 +104,7 @@ pub fn key_changes_proof_check<S: RootsStorage<H>, H: Hasher>(
 	max: u64,
 	key: &[u8]
 ) -> Result<Vec<(u64, u32)>, String> where H::Out: HeapSizeOf {
-	let mut proof_db = MemoryDB::<H>::default();	// TODO: use new for correctness
+	let mut proof_db = MemoryDB::<H>::default();
 	for item in proof {
 		proof_db.insert(&item);
 	}

substrate/core/state-machine/src/changes_trie/prune.rs

@@ -62,7 +62,7 @@ pub fn prune<S: Storage<H>, H: Hasher, F: FnMut(H::Out)>(
 	};
 
 	// delete changes trie for every block in range
-	// TODO: limit `max_digest_interval` so that this cycle won't involve huge ranges
+	// FIXME: limit `max_digest_interval` so that this cycle won't involve huge ranges
 	for block in first..last+1 {
 		let root = match storage.root(current_block, block) {
 			Ok(Some(root)) => root,