MDL-63677 core_user: Avoid redirection during signup

Purge cache just for the current user to avoid redirection when 2
simultaneous users try to sign up at the same time and some policy
has to be agreed.
The 'createduser' invalidation event has been removed also because
is not used any more.
Thanks John Azinheira for spotting it!

lib/db/caches.php

@@ -382,9 +382,6 @@ $definitions = array(
         'mode' => cache_store::MODE_SESSION,
         'simplekeys' => true,
         'simpledata' => true,
-        'ttl' => 1800,
-        'invalidationevents' => array(
-            'createduser',
-        )
+        'ttl' => 1800
     ),
 );

user/lib.php

@@ -126,8 +126,9 @@ function user_create_user($user, $updatepassword = true, $triggerevent = true) {
         \core\event\user_created::create_from_userid($newuserid)->trigger();
     }
 
-    // Purge the associated caches.
-    cache_helper::purge_by_event('createduser');
+    // Purge the associated caches for the current user only.
+    $presignupcache = \cache::make('core', 'presignup');
+    $presignupcache->purge_current_user();
 
     return $newuserid;
 }