core/varlink: several cleanups for metrics varlink server

core/varlink: several cleanups for metrics varlink server

Follow-up for bb1ef2edf7d62de35291702635067ee85f09bad5

The commit introduced a new "metrics" varlink server, but for
user scope stuff it is not bound anywhere. The copy-pasted
"fresh" handling for deserialization is also essentially
meaningless as metrics_setup_varlink_server() doesn't even report
whether the varlink server is fresh (let alone that no serialization
is being done at all right now). Moreover, currently the event
priority is hardcoded, while event loop and associated priority
assignment ought to be subject to each daemon.

While fixing the mentioned issues I took the chance to restructure
the existing code a bit for readability. Note that serialization
for the metrics server is still missing - it will be tackled
in subsequent commits.