dns: custom match rule for DNS records to be tombstoned

dns: custom match rule for DNS records to be tombstoned

A custom match rule for records to be tombstoned by the scavenging process.
Needed because DNS records are a multi-valued attribute on name records, so
without a custom match rule we'd have entire zones into memory to search for
expired records.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=10812

Signed-off-by: Aaron Haslett <aaronhaslett@catalyst.net.nz>
Reviewed-by: Gary Lockyer <gary@catalyst.net.nz>
Signed-off-by: Andrew Bartlett <abartlet@samba.org>