Improve Concurrency and Parallel Request Handling (#19)

Rewrite the swap behaviour so that in-flight requests block process swapping until they are completed. 

Additionally: 

- add tests for parallel requests with proxy.ProxyManager and proxy.Process
- improve Process startup behaviour and simplified the code 
- stopping of processes are sent SIGTERM and have 5 seconds to terminate, before they are killed

proxy/helpers_test.go

@@ -51,7 +51,7 @@ func getTestSimpleResponderConfigPort(expectedMessage string, port int) ModelCon
 
 	// Create a process configuration
 	return ModelConfig{
-		Cmd:           fmt.Sprintf("%s --port %d --respond '%s'", binaryPath, port, expectedMessage),
+		Cmd:           fmt.Sprintf("%s --port %d --silent --respond %s", binaryPath, port, expectedMessage),
 		Proxy:         fmt.Sprintf("http://127.0.0.1:%d", port),
 		CheckEndpoint: "/health",
 	}