When we purge the request cache, also force PHP to collect cycles

Summary:
Ref T12997. See that task for more details. Briefly, an unusual dataset (where commits are mentioned hundreds of times by other commits) is causing some weird memory behavior in the daemons.

Forcing PHP to GC cycles explicitly after each task completes seems to help with this, by cleaning up some of the memory between tasks. A more thorough fix might be to untangle the `$xactions` structure, but that's significantly more involved.

Test Plan:
  - Did this locally in a controlled environment, saw an immediate collection of a 500MB `$xactions` cycle.
  - Put a similar change in production, memory usage seemed less to improve. It's hard to tell for sure that this does anything, but it shouldn't hurt.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T12997

Differential Revision: https://secure.phabricator.com/D18657

src/applications/cache/PhabricatorCaches.php

@@ -51,6 +51,12 @@ final class PhabricatorCaches extends Phobject {
    */
   public static function destroyRequestCache() {
     self::$requestCache = null;
+
+    // See T12997. Force the GC to run when the request cache is destroyed to
+    // clean up any cycles which may still be hanging around.
+    if (function_exists('gc_collect_cycles')) {
+      gc_collect_cycles();
+    }
   }