Stop iroh when it is not used

[link2xt]

To make sure that Iroh is not using the network and e.g. does not measure the connection to relay, we need to stop Iroh when it is not used.

@flub started doing it at https://github.com/chatmail/core/tree/flub/peer-channels

Another related issue is that apps that don't need Iroh use mDNS:
#6443