dns: do not rely on server data structures in code called on client
Replace code which references the DNSRecord and dnsrecord classes with
equivalent code which uses only generic data structures.
This will make it possible to move client code to ipaclient without
dnsrecord bits, DNSRecord and all its subclasses.
The conversion from record value to structured record can't be done on the
client without DNSRecord and subclasses. Introduce a new internal command
dnsrecord_split_parts to do the job on the server when necessary.
https://fedorahosted.org/freeipa/ticket/4739
Reviewed-By: David Kupka <dkupka@redhat.com>