[#1428] Allow non-unique IPs in MySQL and PgSQL

Introduced new host API function which allows for configuring selected
backends to accept non-unique IP reservations for multiple hosts. Support
for it was added in MySQL, Postgres and Kea config file. It is not
supported in Cassandra. New migrations for MySQL and Postgres have been
created.

src/share/database/scripts/pgsql/dhcpdb_create.pgsql

@@ -1015,6 +1015,33 @@ UPDATE schema_version
 -- Commit the script transaction
 COMMIT;
 
+-- Starting from this version we allow specifying multiple IP reservations
+-- for the same address in certain DHCP configurations. The server may check
+-- uniqueness of the IP addresses on its own. This is no longer checked at
+-- the database level to faciliate the use cases when a single host may
+-- get the same reserved IP address via different interfaces.
+
+-- Replace the unique index with non-unique index so the queries for
+-- hosts by IPv4 address are still efficient.
+DROP INDEX IF EXISTS key_dhcp4_ipv4_address_subnet_id;
+CREATE INDEX key_dhcp4_ipv4_address_subnet_id
+    ON hosts (ipv4_address ASC, dhcp4_subnet_id ASC);
+
+-- Replace the unique index with non-unique index so the queries for
+-- hosts by IPv6 address are still efficient.
+ALTER TABLE ipv6_reservations DROP CONSTRAINT IF EXISTS key_dhcp6_address_prefix_len;
+CREATE INDEX key_dhcp6_address_prefix_len
+    ON ipv6_reservations (address ASC, prefix_len ASC);
+
+-- Update the schema version number
+UPDATE schema_version
+    SET version = '6', minor = '2';
+
+-- Schema 6.2 specification ends here.
+
+-- Commit the script transaction
+COMMIT;
+
 -- Notes:
 
 -- Indexes