Don't spend ridiculous amounts of time rebuilding orderable columns

Summary:
Ref T8575. Because orderable columns need to build custom fields, they are relatively expensive to build. Use the request cache.

(The request cache is technically more correct than `static`, because configuration may change between requests and we may eventually reuse interpreters.)

Test Plan: Saw home page time drop 39% (from 462ms to 283ms).

Reviewers: btrahan, avivey

Reviewed By: avivey

Subscribers: avivey, epriestley

Maniphest Tasks: T8575

Differential Revision: https://secure.phabricator.com/D13322

src/infrastructure/query/policy/PhabricatorCursorPagedPolicyAwareQuery.php

···
    * @task order
    */
   public function getOrderableColumns() {
+    $cache = PhabricatorCaches::getRequestCache();
+    $class = get_class($this);
+    $cache_key = 'query.orderablecolumns.'.$class;
+
+    $columns = $cache->getKey($cache_key);
+    if ($columns !== null) {
+      return $columns;
+    }
+
     $columns = array(
       'id' => array(
         'table' => $this->getPrimaryTableAlias(),
···
         );
       }
     }
+
+    $cache->setKey($cache_key, $columns);
 
     return $columns;
   }