FRM/Statistics Manager: figure out flow indexing

Currently we do not have a way how statistics manager can cross-reference a flow stats report to a preconfigured flow.

This results in org.opendaylight.controller.md.statistics.manager.FlowStatsTracker needing to go through entire configured flow table on each flow statistics report, which simply does not scale.

FRM needs to enforce non-existing cookie, then expose a cookie->FlowKey mapping in datastore. Then SM can lookup the FlowKey based on cookie, and Flow from FlowKey.