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/hrmp.md

@@ -4,14 +4,6 @@ A module responsible for Horizontally Relay-routed Message Passing (HRMP). See [
 
 ## 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>;
-```
-
 HRMP related structs:
 
 ```rust
@@ -171,11 +163,6 @@ Candidate Enactment:
     > If that becomes a problem consider introducing an extra dictionary which says at what block the given sender
     > sent a message to the recipient.
 
-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.
-
 ## Entry-points
 
 The following entry-points are meant to be used for HRMP channel management.
@@ -241,7 +228,7 @@ the parachain executed the message.
 
 ## 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 all inbound channels of `P`, i.e. `(_, P)`,
     1. Remove all outbound channels of `P`, i.e. `(P, _)`,
     1. Remove `HrmpOpenChannelRequestCount` for `P`