TCP timeout opt-in instead of opt-out

For speed, idle connections are super cheap.

lib/net.js

@@ -145,6 +145,7 @@ var timeout = new (function () {
 
 
   var unenroll = this.unenroll = function (socket) {
+    if (socket._idleNext) {
       socket._idleNext._idlePrev = socket._idlePrev;
       socket._idlePrev._idleNext = socket._idleNext;
 
@@ -155,6 +156,7 @@ var timeout = new (function () {
         //sys.puts('unenroll: list empty');
         list.stop();
       }
+    }
   };
 
 
@@ -256,8 +258,6 @@ function _doFlush () {
 }
 
 function initSocket (self) {
-  timeout.enroll(self, 60*1000); // default timeout, 60 seconds
-
   self._readWatcher = ioWatchers.alloc();
   self._readWatcher.callback = function () {
     // If this is the first recv (recvBuffer doesn't exist) or we've used up