Review swarm name for local swarm discovery

[Frando]

For local swarm discovery (mDNS) we currently hard-code the service name passed to swarm-discovery to iroh.local.swarm.
With the way swarm-discovery encodes things, this results in a mDNS query domain of _iroh.local.swarm._udp.local., and response records of the form f4neyiri6azvb73uyprghwk5fbpmjtwsly32x4mv2ez3scg4ft5a._iroh.local.swarm._udp.local.

IMO, the local.swarm serves no purpose and makes the DNS packets needlessly longer. The default service name could just be iroh, so that the search domain would be _iroh._udp.local.

This would be a wire breaking change so if we want to do this, it should be done together with our other wire breaking changes.

Additionally, I think we should make the service name configurable.

[flub]

Are you sure this has to be fully wire-breaking? Can we not configure the discovery service to use both of those query domains? That allows transitioning a bit nicer.

In general, even for our wire-breaking changes we want to support both old and new versions before 1.0. And only at some 1.0-betaX or something version drop the old version. This would give folks a version of iroh that can work with both the 1.0 iroh and previous iroh to help them navigate the wire-breaking changes.

Wire breaking changes which are truly either-or are really bad and we should avoid as much as we can.