mod_s2s switched to net.connect for outgoing connections in https://hg.prosody.im/trunk/rev/756b8821007a
Previously, s2sout.lib would maintain one instance of a DNS resolver per connection attempt.
With net.connect, net.resolver.service creates one DNS resolver instance,
then for each SRV target another DNS resolver is created by net.resolvers.basic.
Not a problem anymore with the switch to libunbound