Implementer's guide: downward messages and HRMP, take 2 (#1503)

* First stab at downward messages.

That also includes a notion of horizontal messages.

* Add some structure to the router.

* Update `ValidationOutputs`

* Add `processed_downward_messages` to `ValidationOutputs`.

Forgot to check that in.

* s/AccountId/ParaId

* DownwardMessage::ParachainSpecfic

* s/ensure_horizontal_messages_fits/ensure_horizontal_messages_fit

* Clarify that Router called for each candidate

* Update the preamble for Router.

* Rewrite the relay-chain extrinsic routines

* Update gloassary

* Add DMP to the glossary

* If the queue is empty, `processed_downward_messages` can be 0

* WIP

* Add condemned list

* Pivot to message-storing channel based HRMP

* Finished draft

* Tidy up

* Remove a duplicate glossary entry

* Fix typo

* Fix wording to emphasize that the channel is unidirectional

* Proper decrement `HrmpOpenChannelRequestCount`

* Add a comment for `HrmpOpenChannelRequestCount`.

* Remove old configuration values.

* Be more specific about the para{chain,thread} hrmp chan limits.

* Fix indentation so the lists are rendendered properly

* "to answer **the**" question instead of "a"

* Add a missing call to `check_processed_downward_messages`

* Clean more stuff during offboarding

* Fix typo

* Fix typo for the config

* Add a call to `prune_dmq`

* Add explicit invariants for ingress/egress indexes

* Add comments for the sender/reciever deposit config fields

* Document various fields and structs in Router module

* More docs

* Missing docs in Candidate.md

* Tabs to spaces in router.md

* Apply Rob's suggestion

* Add the hrmp_ prefix to the router messages

* Those are entry points

* Use SessionIndex type for the `age` field

* Use a struct to represent `HrmpChannelId`

* Put only MQCs into the LocalValidationData

* Close request can be initiated by the runtime directly

* Close request can be initiated by the runtime directly

* tabs/spaces

* Maintain the list of the outgoing paras in Router

* Update roadmap/implementers-guide/src/runtime/inclusion.md

Co-authored-by: Robert Habermeier <[email protected]>

* fix typo

* Remove an unnecessary pair of code quotes

Co-authored-by: Robert Habermeier <[email protected]>