Print "stalled" task on shutdown (#13022)

* Print "stalled" task on shutdown

When the node is shutting down, we give the Tokio runtime 60 seconds to shutdown. If after these 60
seconds there are still running tasks, we now print these tasks. This should help debugging nodes
that have stalled tasks.

This pr introduces a `TaskRegistry` that keeps track of all running tasks. Each task registers and
unregisters itself in this `TaskRegistry`.

* Fix rustdoc

* Update client/service/src/lib.rs