MDL-74911 assignfeedback_editpdf: unlock session when polling conversion

Polling conversions will run the document conversion immediately if the
conversion has not already been completed.

Releasing the session lock while doing this ensures large conversions
do not unnecessarily hold the users session.

mod/assign/feedback/editpdf/ajax.php

@@ -52,6 +52,9 @@ if (!$assignment->can_view_submission($userid)) {
 }
 
 if ($action === 'pollconversions') {
+    // Poll conversions does not require session lock.
+    \core\session\manager::write_close();
+
     $draft = true;
     if (!has_capability('mod/assign:grade', $context)) {
         // A student always sees the readonly version.