we need an interface to query stats of mme/sgw/pgw

[spencersevilla]

This could be done several different ways, but we definitely need some sort of method for reading the MME/SGW/PGW stats on demand. Could use an IPC socket, could use UDP/TCP, could use a REST API, doesn't really matter to me. Would like to see this done ASAP because this is a very useful debugging "multitool" (i.e. if there are 0 connected eNB or UE, we've got a problem even if the logs don't say so).