PostgreSQL corrections for schema 7.0

revert dhcp4_subnet_id to INT for the initial creation. It is already altered to BIGINT on dhcpdb_create.pgsql:542 and upgrade_3.2_to_3.3.sh.in:44.
make upgrade_6.2_to_7.0.sh.in consistent with dhcpdb_create.pgsql

Other less important changes:

CREATE OR REPLACE everywhere, just as it was recently modified in dhcpdb_create.pgsql
add INSERT INTO host_identifier_type VALUES (3, 'client-id'); to upgrade script. Not related to changes made in #90 (closed) and #2166 (closed), but it is missing since 2016. I would prefer it in its intended upgrade script.

Edited Nov 23, 2021 by Andrei Pavel

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