Add unique constraints in IPAvailabilityRange

first_ip, allocation_pool_id and last_ip, allocation_pool_id
should be unique in the table.
These constraints are essential to detect concurrent modifications
of the IpAvailabilityRange table if the SELECT ... FOR UPDATE
lock is removed

Change-Id: Iaf2288c0b6bf27e93c03691073d7f505ef24fdd3
Closes-bug: #1373015
Merge "Add unique constraints in IPAvailabilityRange"