Do not spin on aceept() with EMFILE

When a server hit EMFILE it would continue to try to accept new connections
from the queue. This patch introduces a timeout of one second where it will
stop trying to accept new files. After the second is over it tries again.

This is a rather serious bug that has been effecting many highly concurrent
programs. It was introduced in 4593c0, version v0.2.0.

TODO: A test for this situation. Currently I test it like this

  termA% cd projects/node
  termA% ulimit -n 256
  termA% ./node benchmark/idle_server.js

  termB% cd projects/node
  termB% ./node benchmark/idle_clients.js

And watch how the server process behaves.

TODO

@@ -25,3 +25,4 @@
   based on size but rather read until EOF into a chain of buffers, then 
   concat them together.
 - process object should be defined in src/node.js not in c++
+- Test for EMFILE accept spin bug.

lib/net.js

@@ -1049,6 +1049,25 @@ function Server (listener) {
 
   self.connections = 0;
 
+  self.paused = false;
+  self.pauseTimeout = 1000;
+
+  function pause () {
+    // We've hit the maximum file limit. What to do?
+    // Let's try again in 1 second.
+    self.watcher.stop();
+
+    // If we're already paused, don't do another timeout.
+    if (self.paused) return;
+
+    setTimeout(function () {
+      self.paused = false;
+      // Make sure we haven't closed in the interim
+      if (typeof self.fd  != 'number') return;
+      self.watcher.start();
+    }, self.pauseTimeout);
+  }
+
   self.watcher = new IOWatcher();
   self.watcher.host = self;
   self.watcher.callback = function () {
@@ -1056,7 +1075,10 @@ function Server (listener) {
       try {
         var peerInfo = accept(self.fd);
       } catch (e) {
-        if (e.errno == EMFILE) return;
+        if (e.errno == EMFILE) {
+          pause();
+          return;
+        }
         throw e;
       }
       if (!peerInfo) return;