When running gvmd with the --optimize=cleanup-sequences option, the
database checks during initialization will avoid most inserts.
Setting database_version and max_hosts in the meta table now uses
INSERT ... ON CONFLICT instead of deleting the previous entry.

Skipping the inserts prevents the cleanup failing if one of the
sequence has already run out of ids.
The change to the meta table update reduces the risk of the meta table
id sequence running out.

The new type "krb5" is added to the create_credential, modify_credential
and get_credentials commands.

This will later be usable in scan targets for Kerberos 5 authentication.

The new parameter has been added to the check_db_port_lists prototype
and various check_db_... functions that would only return if
the avoid_db_check_inserts parameter is true have the parameter removed
and are instead wrapped in if conditions.