cluster: fix O(n*m) scan of cmd string

Don't scan the whole string for a "NODE_CLUSTER_" substring, just check
that the string starts with the expected prefix. The linear scan was
causing a noticeable (but unsurprising) slowdown on messages with a
large .cmd string property.

lib/cluster.js

@@ -141,9 +141,10 @@ cluster.setupMaster = function(options) {
 // Check if a message is internal only
 var INTERNAL_PREFIX = 'NODE_CLUSTER_';
 function isInternalMessage(message) {
-  return (isObject(message) &&
+  return isObject(message) &&
           typeof message.cmd === 'string' &&
-          message.cmd.indexOf(INTERNAL_PREFIX) === 0);
+          message.cmd.length > INTERNAL_PREFIX.length &&
+          message.cmd.slice(0, INTERNAL_PREFIX.length) === INTERNAL_PREFIX;
 }
 
 // Modify message object to be internal