cluster: ignore queryServer msgs on disconnection

It avoids the creation of unnecessary handles. This issue is causing
intermitent failures in `test-cluster-disconnect-race` on `FreeBSD`
and `OS X`:

```
assert.js:89
  throw new assert.AssertionError({
  ^
AssertionError: Resource leak detected.
    at removeWorker (cluster.js:321:7)
    at ChildProcess.<anonymous> (cluster.js:356:9)
    at ChildProcess.g (events.js:264:16)
    at emitTwo (events.js:88:13)
    at ChildProcess.emit (events.js:173:7)
    at Process.ChildProcess._handle.onexit (internal/child_process.js:200:12)
```

The problem is that the `worker2.disconnect` is being called on the
master before the `queryServer` is handled, causing the worker to
be deleted, then the Server handle is created afterwards. Later on,
when `removeWorker` is called from the `exit` handler, there are no
workers left, but one handle, thus the `AssertionError`.

Add a new `test/sequential/test-cluster-disconnect-leak` based on
`test-cluster-disconnect-race` that creates lots of workers and fails
consistently without this patch.

lib/cluster.js

@@ -446,6 +446,9 @@ function masterInit() {
   }
 
   function queryServer(worker, message) {
+    // Stop processing if worker already disconnecting
+    if (worker.suicide)
+      return;
     var args = [message.address,
                 message.port,
                 message.addressType,

test/sequential/test-cluster-disconnect-leak.js

@@ -0,0 +1,47 @@
+'use strict';
+// Flags: --expose-internals
+
+const common = require('../common');
+const assert = require('assert');
+const net = require('net');
+const cluster = require('cluster');
+const handles = require('internal/cluster').handles;
+const os = require('os');
+
+if (common.isWindows) {
+  console.log('1..0 # Skipped: This test does not apply to Windows.');
+  return;
+}
+
+cluster.schedulingPolicy = cluster.SCHED_NONE;
+
+if (cluster.isMaster) {
+  const cpus = os.cpus().length;
+  const tries = cpus > 8 ? 128 : cpus * 16;
+
+  const worker1 = cluster.fork();
+  worker1.on('message', common.mustCall(() => {
+    worker1.disconnect();
+    for (let i = 0; i < tries; ++ i) {
+      const w = cluster.fork();
+      w.on('online', common.mustCall(w.disconnect));
+    }
+  }));
+
+  cluster.on('exit', common.mustCall((worker, code) => {
+    assert.strictEqual(code, 0, 'worker exited with error');
+  }, tries + 1));
+
+  process.on('exit', () => {
+    assert.deepEqual(Object.keys(cluster.workers), []);
+    assert.strictEqual(Object.keys(handles).length, 0);
+  });
+
+  return;
+}
+
+var server = net.createServer();
+
+server.listen(common.PORT, function() {
+  process.send('listening');
+});