ipdb: [regression fixed] do not lookup master in cache

This way it will always present, even fot detached interfaces, since the
cache is not updated yet.
ipdb: cleanup detached interfaces
ipdb: use absolute values to detect LinkedSet status

Do not count LinkedSet updates, but check the absolute threshold value.
This way LinkedSet can ignore multiple add/delete call of the same item,
notifying watchers only when threshold is reached.