The group members cache can quickly end up being filled with users as key

When loading a group, getMembers is also used for each member of the group, in case it's a group. The problem is that it means the cache can quickly get filled with users and push the groups themself out, making the cache quite useless when there is a lot of users.

To limit this effect, and as a bonus provide a new feature to the GroupManager, it would be interesting to introduce an isGroup(DocumentReference) and its corresponding cache which would be in charge of maintaining a big cache of boolean indicating if an entry is a group or not.

The workaround to be safe is to set the size of the "user.membership.members" to at least (the number of users + the number of groups) x 2.