Improve shutdown behaviour (#47) (#49)

Introduce `Process.Shutdown()` and `ProxyManager.Shutdown()`. These two function required a lot of internal process state management refactoring. A key benefit is that `Process.start()` is now interruptable. When `Shutdown()` is called it will break the long health check loop. 

State management within Process is also improved. Added `starting`, `stopping` and `shutdown` states. Additionally, introduced a simple finite state machine to manage transitions.

llama-swap.go

@@ -47,7 +47,7 @@ func main() {
 	go func() {
 		<-sigChan
 		fmt.Println("Shutting down llama-swap")
-		proxyManager.StopProcesses()
+		proxyManager.Shutdown()
 		os.Exit(0)
 	}()