Add a send_request function to NetworkService (#8008)

* Add a `send_request` to `NetworkService`.

This function delivers responses via a provided sender and also allows
for sending requests to currently not connected peers.

* Document caveats of send_request better.

* Fix compilation in certain cases.

* Update docs + introduce IfDisconnected enum

for more readable function calls.

* Doc fix.

* Rename send_request to detached_request.

* Whitespace fix - arrrgh

* Update client/network/src/service.rs

spaces/tabs

Co-authored-by: Pierre Krieger <[email protected]>

* Update client/network/src/request_responses.rs

Documentation fix

Co-authored-by: Roman Borschel <[email protected]>

* Update client/network/src/service.rs

Typo.

Co-authored-by: Roman Borschel <[email protected]>

* Update client/network/src/service.rs

Better docs.

Co-authored-by: Roman Borschel <[email protected]>

* Update client/network/src/service.rs

Typo.

Co-authored-by: Roman Borschel <[email protected]>

* Update client/network/src/service.rs

Doc improvements.

Co-authored-by: Roman Borschel <[email protected]>

* Remove error in logs on dialing a peer.

This is now valid behaviour.

* Rename detached_request to start_request.

As suggested by @romanb.

* Fix merged master.

* Fix too long lines.

Co-authored-by: Pierre Krieger <[email protected]>
Co-authored-by: Roman Borschel <[email protected]>