dgram: avoid EventEmitter leak warning

When a datagram socket hasn't been bound yet, node will defer `send()`
operations until binding has completed. Before this patch a `listening`
listener would be installed every time `send` was called. This triggered
an EventEmitter leak warning when more than 10 packets were sent in a
tight loop. Therefore switch to using a single `listening` listener, and
use an array to enqueue outbound packets.

lib/dgram.js

@@ -243,10 +243,21 @@ Socket.prototype.send = function(buffer,
   if (self._bindState == BIND_STATE_UNBOUND)
     self.bind(0, null);
 
+  // If the socket hasn't been bound yet, push the outbound packet onto the
+  // send queue and send after binding is complete.
   if (self._bindState != BIND_STATE_BOUND) {
-    self.once('listening', function() {
-      self.send(buffer, offset, length, port, address, callback);
-    });
+    // If the send queue hasn't been initialized yet, do it, and install an
+    // event handler that flushes the send queue after binding is done.
+    if (!self._sendQueue) {
+      self._sendQueue = [];
+      self.once('listening', function() {
+        // Flush the send queue.
+        for (var i = 0; i < self._sendQueue.length; i++)
+          self.send.apply(self, self._sendQueue[i]);
+        self._sendQueue = undefined;
+      });
+    }
+    self._sendQueue.push([buffer, offset, length, port, address, callback]);
     return;
   }