Ensure that handshake is sent back even in case of back-pressure (#6979)

* 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 <mail@max-inden.de>

* Also process OpenRequest and Closed

* Fix bad merge

* God I'm so lost with all these merges

* Immediately return Closed

Co-authored-by: Max Inden <mail@max-inden.de>