Add data model for host reservations

We need a data model in the Stork database to store host reservations. Multiple reservations may appear for a single host and the host may be identified in various ways, e.g. using MAC address, using DHCP specific values such as client identifier or anything else. This ticket adds such model to the database. The reservations should initially include IP addresses and delegated prefixes. We don't store DHCP options at this point for host reservations.