Stop sending messages on legacy substream altogether (#6975)

* Stop sending messages on legacy substream altogether

* Ensure that handshake is sent back even in case of back-pressure

* Update client/network/src/protocol/generic_proto/handler/group.rs

Co-authored-by: Max Inden <[email protected]>

* Also process OpenRequest and Closed

* Also process OpenRequest and Closed

* Fix bad merge

* God I'm so lost with all these merges

* Immediately return Closed

* Add warning for sending on non-registered protocol

* Register GrandPa protocol in tests

* Update client/network/src/protocol/generic_proto/handler/group.rs

Co-authored-by: Max Inden <[email protected]>

Co-authored-by: Max Inden <[email protected]>