Reject connections from xmr-node-proxy

xmr-node-proxy is not supported and crashes with an error when it
connects to pool that does not support it. This makes it crash in a
cleaner way.

Fixes: GH-6

src/main/java/uk/offtopica/moneropool/stratum/StratumServerHandler.java

@@ -56,7 +56,10 @@ private void onLogin(ChannelHandlerContext ctx, StratumRequest request) {
         final String agent;
         if (params.containsKey("agent")) {
             agent = (String) params.get("agent");
-            // TODO: Check for xmr-node-proxy (tell them to go away).
+            if (agent.startsWith("xmr-node-proxy")) {
+                replyWithError(ctx, request.getId(), new StratumError(-1, "xmr-node-proxy is not supported"));
+                return;
+            }
         } else {
             agent = null;
         }