Add metering infrastructure

 * Defines a MeterTag marker trait for FlowTags.
 * Makes the FlowController & FlowManager track those tags as meters.
   that get stats from the flows that have them.
 * Adds a JMX interface to these meters.
 * Adds a command line tool to query them ala vmstat.

Change-Id: I105b896de7a674d7cd54252fd1c49343ea246bed
Signed-off-by: Guillermo Ontanon <guillermo@midokura.jp>
Merge "Add metering infrastructure"