sd-dhcp6-lease: Add DHCPv6 lease handling

sd-dhcp6-lease: Add DHCPv6 lease handling

Create a structure describing a DHCPv6 lease. Add internal functions
for creating a new lease and accessing the server ID, preference and
IAID. Provide functions for clearing addresses and associated timers.

External users are initially given only the capabilities of
referencing and unreferencing the lease structure.