varnish: adapt to metrics renames & deprecations in varnish4

varnish: adapt to metrics renames & deprecations in varnish4

Summary of changes:
- connections: "accepted" & "dropped" are now found in the session section
- dirdns: doesn't exist in varnish 4 anymore
- objects: "n_obj*" were removed from varnish 4
- ban: "obj", "req" & "completed" bans were added. As a lot of new
  "tested" metrics are available, so we only collectd the overall total
  now
- session: metrics from "connections" and "threads" categories were
  moved here
- struct: "n_sess*" were removed
- totals: new, more detailed *bytes metrics were made available
- threads: new metrics were made available. "queued" was moved the
  session section.