unbound-dhcp-bridge: Rewrite update algorithm

unbound-dhcp-bridge: Rewrite update algorithm

Before the bridge tries reading any existing leases from unbound
but this makes it difficult to destinguish between what is a DHCP lease,
static host entry or anything else.

This patch will change the bridge back to just remember what has been
added to the cache already which makes it easier to keep track.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>