minor symfony#24913 HttpCache lock update (Felix Marezki)

This PR was merged into the 3.4 branch.

Discussion
----------

HttpCache lock update

simplification...
I guess the original Author was trying to pursue something like a martingale (doubling) strategy (or in this case the increment over a constant factor) but well ... he did not. Instead a constant wait time was introduced, which means the wait is simply executed a 100 times over (in the worst case). And so I do not see a need for "5000000" and "+= 50000" to remain here.

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

<!--
- Bug fixes must be submitted against the lowest branch where they apply
  (lowest branches are regularly merged to upper ones so they get the fixes too).
- Features and deprecations must be submitted against the 3.4,
  legacy code removals go to the master branch.
- Please fill in this template according to the PR you're about to submit.
- Replace this comment by a description of what your PR is solving.
-->

Commits
-------

8464bd0 HttpCache lock update

Author: fabpot

src/Symfony/Component/HttpKernel/HttpCache/HttpCache.php

@@ -36,6 +36,8 @@ class HttpCache implements HttpKernelInterface, TerminableInterface
     private $traces = array();
 
     /**
+     * Constructor
+     *
      * The available options are:
      *
      *   * debug:                 If true, the traces are added as a HTTP header to ease debugging
@@ -702,11 +704,11 @@ private function mayServeStaleWhileRevalidate(Response $entry)
     private function waitForLock(Request $request)
     {
         $wait = 0;
-        while ($this->store->isLocked($request) && $wait < 5000000) {
+        while ($this->store->isLocked($request) && $wait < 100) {
             usleep(50000);
-            $wait += 50000;
+            ++$wait;
         }
 
-        return $wait < 5000000;
+        return $wait < 100;
     }
 }