Some code cleanup in overseer (#2008)

* Some code cleanup in overseer

- Switches to select! in the overseer run loop to be more fair about
message processing between the different sources.
- Added a check to only send `ActiveLeaves` if the update actually
contains any data.

* Move the check

* Restore old behavior

* Simplify message sending and signal sending to subsystems

* Update node/subsystem/src/lib.rs