Initializer + Paras Clean Up Messages When Offboarding (#2413)

* initial hack in

* finish up

* use notification to pass outgoing paras

* move outgoing paras from notifications

* missing comma

* update guides

* clean up

polkadot/roadmap/implementers-guide/src/runtime/ump.md

@@ -4,14 +4,6 @@ A module responsible for Upward Message Passing (UMP). See [Messaging Overview](
 
 ## Storage
 
-General storage entries
-
-```rust
-/// Paras that are to be cleaned up at the end of the session.
-/// The entries are sorted ascending by the para id.
-OutgoingParas: Vec<ParaId>;
-```
-
 Storage related to UMP
 
 ```rust
@@ -70,11 +62,6 @@ Candidate Enactment:
         1. Increment the size and the count in `RelayDispatchQueueSize` for `P`.
         1. Ensure that `P` is present in `NeedsDispatch`.
 
-The following routine is intended to be called in the same time when `Paras::schedule_para_cleanup` is called.
-
-`schedule_para_cleanup(ParaId)`:
-    1. Add the para into the `OutgoingParas` vector maintaining the sorted order.
-
 The following routine is meant to execute pending entries in upward message queues. This function doesn't fail, even if
 dispatcing any of individual upward messages returns an error.
 
@@ -92,7 +79,7 @@ dispatcing any of individual upward messages returns an error.
 
 ## Session Change
 
-1. Drain `OutgoingParas`. For each `P` happened to be in the list:.
+1. For each `P` in `outgoing_paras` (generated by `Paras::on_new_session`):
     1. Remove `RelayDispatchQueueSize` of `P`.
     1. Remove `RelayDispatchQueues` of `P`.
     1. Remove `P` if it exists in `NeedsDispatch`.