Use sdnotify for systemd (#768)

* Use sdnotify for systemd

This is useful if a configuration is long to load.
Systemd will continue dependency chain only when server have finish to start.

https://www.freedesktop.org/software/systemd/man/systemd.service.html#Type=

* Extract the waiting behavior from Start()

server.go

@@ -91,7 +91,7 @@ func NewServer(globalConfiguration GlobalConfiguration) *Server {
 	return server
 }
 
-// Start starts the server and blocks until server is shutted down.
+// Start starts the server.
 func (server *Server) Start() {
 	server.startHTTPServers()
 	server.startLeadership()
@@ -104,6 +104,10 @@ func (server *Server) Start() {
 	server.configureProviders()
 	server.startProviders()
 	go server.listenSignals()
+}
+
+// Wait blocks until server is shutted down.
+func (server *Server) Wait() {
 	<-server.stopChan
 }