Previously we had to acquire a lock whenever we called Mark() on any metric,
since metrics like ingress and egress are marked on every arriving packet
there was a lot of time spent on acquiring their locks. This commit changes
the semantics of a standard meter such that we write into a temporary buffer
and add the buffer to the metrics every 5 seconds, which reduces the computation
needed by metrics significantly.

This change also has some further implications. Rates are not as stable as they
used to be, since we update the meters only once every 5 seconds not whenever
the meter was marked. This means we have to update tests that need *instant*
finality of the marked meter.