refactor: Transaction-Payment module (#3816)

* Initial draft that compiles

* Extract payment stuff from balances

* Extract multiplier update stuff from system

* Some fixes.

* Update len-fee as well

* some review comments.

* Remove todo

* bump

substrate/srml/finality-tracker/src/lib.rs

@@ -298,7 +298,6 @@ mod tests {
 		type AccountId = u64;
 		type Lookup = IdentityLookup<u64>;
 		type Header = Header;
-		type WeightMultiplierUpdate = ();
 		type Event = ();
 		type BlockHashCount = BlockHashCount;
 		type MaximumBlockWeight = MaximumBlockWeight;