Automatic member number management

You define for the user status, if this has a membership number or not. For example, new members and regular members do have a membership number, but former members do not have. Currently, the membership number management is very simple: the membership number start at 1 and is increased by each member. When a member leaves, its membership number will be blocked (not used) for some time, by default 6 months. After that, the next new member will "recycle" this membership number.