dns: dns record scavenging function (without task)

dns: dns record scavenging function (without task)

DNS record scavenging function with testing.  The logic of the custom match rule
in previous commit is inverted so that calculations using zone properties can
be taken out of the function's inner loop. Periodic task to come.

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>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>