IPv6 support in Postfilter

The current version of Postfilter is 2.4, which does not support IPv6 yet. The missing parts which will be fixed in the next release are

Some table fields are optimized for IPv4 addresses: the field width must be increased to accomodate IPv6 addresses
- greylist table, _ip field
- report table, _client_address field
Updating tables automatically with the IP(v4) address of the client supports to update just the given netblock of the client, which supports IPv4 only. The algorithm is to be extended to support IPv6.
The Postfilter::DNS module does not support AAAA records.

With these parts fixed, Postfilter will fully support IPv6.