Add connection webhook notifications (#392)

Also 

* Added decode of LoginStart message
* Add metrics backend constants
* Updated usage section
* Documented MaxFrameLength

server/notifier.go

@@ -0,0 +1,19 @@
+package server
+
+import (
+	"context"
+	"net"
+)
+
+type ConnectionNotifier interface {
+	// NotifyMissingBackend is called when an inbound connection is received for a server that does not have a backend.
+	NotifyMissingBackend(ctx context.Context, clientAddr net.Addr, server string, playerInfo *PlayerInfo) error
+
+	// NotifyFailedBackendConnection is called when the backend connection failed.
+	NotifyFailedBackendConnection(ctx context.Context,
+		clientAddr net.Addr, serverAddress string, playerInfo *PlayerInfo, backendHostPort string, err error) error
+
+	// NotifyConnected is called when the backend connection succeeded.
+	NotifyConnected(ctx context.Context,
+		clientAddr net.Addr, serverAddress string, playerInfo *PlayerInfo, backendHostPort string) error
+}