Long story short: Once we create a server and start it, we can't start
it again. We can't close it and restart it, and we can't just start
it without closing it.
Unfortunately, if the server's config needs to change, we have a Problem
here.
This ultimately means that the retry logic for GetListeners can't actually
retry successfully; if we fail on the first attempt, we necessarily fail
on any later attempts also, and if we try to fix that, we get panics.
But!
We don't actually NEED to retry. We just need to ensure that we can
open a :0 port, extract the actual port number, and use that in places
where the port number mattered, without having to rebind it.
The only actual place we needed to rebind things was opening gRPC
servers, so we introduce a gRPC Listener that can be used instead of
trying to bind to a specified port.
In a bunch of other cases where we had similar logic to try to allocate
and then use a port, we can switch to just using a provided listener.
For instance, net/http has `Serve(net.Listener, handler)`, not just
ListenAndServe(addr, handler).
This should eliminate the weird CI failures from eaddrinuse.
NOT fixed: server/cluster_test.go/TestClusterResize_AddNode isn't working
right now. The new node isn't actually being added to the existing cluster.
I attempted this but was outsmarted by it, and I think fixing the
rest of this is worth it as a separate thing.
