IPVS stats export

[SpComb]

clusterf-ipvs should support monitoring the IPVS service/dest stats from the kernel, and exporting them via some sensible interface such as statsd, influxdb, prometheus etc.

This needs ipvs support for decoding the IPVS_SVC_ATTR_STATS and IPVS_DEST_ATTR_STATS into a stats struct.

The IPVSDriver should support some goroutine-safe way of retrieving the current set of Services/Dests and their originating Config service/backend Node.

Some stats component should then query the IPVS stats periodically from the kernel, and export them by the Config name.

This would have the advantage over than any separate IPVS stats collector that we can associate each IPVS Service and Dest with the named Config node to monitor metrics by the actual service/backend name. The service metrics could be used for e.g. autoscaling purposes, launching more Docker backends based on the IPVS connection load.