VLR: high water mark on subscriber storage
In libvlr, we (will) keep every subscriber in RAM as long as it still has valid auth tuples (3GPP TS 33.102 Annex C.2.3).
Most subscribers will still have auth tuples left upon detaching, so we would store all subscribers.
Implement a configurable way of limiting the number of subscribers kept in RAM, e.g. a fixed max size.
If this is reached, discard those subscribers that have been inactive for the longest time first.
Before the VLR, there were VTY commands to mark a subscriber to be kept in ram, i.e. to not drop it from in-ram structures because we no longer use it, in order to allow continuously querying it from the vty. When we implement subscriber removal, we may also need such mechanisms again.