The invalidation of a modifed group which contains a lot of users can be very slow

According to caubin, the save of the document itself is not really the main problem, but the automatic invalidation of the security cache for all the group members (especially, gathering those group members, just after the save) can take a long time.

To reproduce: write a script which add 10 000 XWiki.XWikiGroups with different members in a XWiki.TestGroup document

Adding the first user took around 100 ms for me, but any new add takes about 30s when the document already contains 10 000 members.

We should try to find ways to optimize this.