Improve domain usage in http_simple benchmark

There were two problems with the way domains were used in the benchmark:

a) A global domain was created. IMO this is not how one would normally
use domains, as an error caught by a global domain would be just as
difficult to handle as an 'uncaughtException' one. More importantly:
Setting up a global domain makes implicit domain member tracking
expensive as per-request domains will now also be tracked as members of
the global domain.

b) The per-request domains created in the benchmark were never entered.
While it didn't have an impact on the performance of the benchmark, it
does seem a bit pointless to not enter the domains, so this patch causes
them to be entered.

benchmark/http_simple.js

@@ -13,22 +13,16 @@ var fixed = makeString(20 * 1024, 'C'),
 
 var useDomains = process.env.NODE_USE_DOMAINS;
 
-// set up one global domain.
 if (useDomains) {
   var domain = require('domain');
-  var gdom = domain.create();
-  gdom.on('error', function(er) {
-    console.log('Error on global domain', er);
-    throw er;
-  });
-  gdom.enter();
 }
 
 var server = http.createServer(function (req, res) {
   if (useDomains) {
     var dom = domain.create();
     dom.add(req);
     dom.add(res);
+    dom.enter();
   }
 
   var commands = req.url.split('/');