Add a back-pressure-friendly alternative to NetworkService::write_notifications (#6692)

* Add NetworkService::send_notifications

* Doc

* Doc

* API adjustment

* Address concerns

* Make it compile

* Start implementation

* Progress in the implementation

* Change implementation strategy again

* More work before weekend

* Finish changes

* Minor doc fix

* Revert some minor changes

* Apply suggestions from code review

* GroupError -> NotifsHandlerError

* Apply suggestions from code review

Co-authored-by: Roman Borschel <romanb@users.noreply.github.com>

* state_transition_waker -> close_waker

* Apply suggestions from code review

Co-authored-by: Roman Borschel <romanb@users.noreply.github.com>

* Finish renames in service.rs

* More renames

* More review suggestsions applied

* More review addressing

* Final change

* 512 -> 2048

Co-authored-by: Roman Borschel <romanb@users.noreply.github.com>