Perfcounters + Endpoint SLA

Added basic support for passing performance counter data via the heartbeat. Added some naive endpoint sla monitoring to demo how the perfcounters can be used in ServiceControl.

@johnsimons if you merge this you can test it by dropping the plugin dll into the fullduplex server. Add a sleep in the handler and when critical time goes > 30 s you see the EndpointSLABreached event fire => use that to notify the UI via signalr