Add locking when several users authenticate at the same time and are part of the same group

We noticed performance issues on simple pages (slowness or 502 error) when several large groups need to be synched, e.g.

• the wiki has a few groups with 15k+ users
• a sync is triggered to update the groups upon accessing a page
  An improvement could be added to perform some locking to ensure that the same group document isn't changed by several threads at the same time, overriding the changes of the other threads.