dhcp: rework static lease logic

dhcp: rework static lease logic

Rework the static lease logic as the existing logic had different issues.
Static leases are now added in a vlist tree which makes it easier to handle
static lease config changes.
For both DHCPv4 and DHCPv6 static assignments are now created upon the
receival of DHCPv4/DHCPv6 messages as before the static assignment was
created even if the client was not physically present.
In case a hostname is specified in a static lease it won't be overriden
anymore by the hostname received from the client.

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>