Update Record object with structured RRdata

Each of the supported record types now has its own object. from_text
and to_text methods convert from/to the attribute 'data'. Storage and
Central now return the actual specific record object i.e. for an A
record type the RRData_A object is returned.
Implements: blueprint mdns-structured-rdata

Change-Id: I3c6e4541903cb19cb33228a1a8c50b2a94f52205
Merge "Update Record object with structured RRdata"