remove connected disconnected state, 3rd attempt (#3898)

* overseer: remove mut in connector

* rename SelectRelayChainWFallback -> SelectRelayChain

* split Basics

* introduce the OverseerConnector, use it

* introduce is_relay_chain to RelayChainSelection

* chore: rename var

* avoid dummy import in subsystem

* actually remove Disconnecte/Connected enum

* extract DummySubsystem into mod dummy.

* Handle::Connected -> Handle::new

* chore: fmt

* fix test

* select relay chain takes no arg, simplification

* fmt

* Update node/service/src/lib.rs

Co-authored-by: Andronik Ordian <write@reusable.software>

* chore: improve malus tests

* avoid the deferred setting of `is_relay_chain` in `RelayChainSelection`

* positive assertion is not mandated, only the negative one, to avoid a stall

* chore: fmt

* assure the `RelayChainSelection` is not used before the overseer is up and running

Co-authored-by: Andronik Ordian <write@reusable.software>