entry: dnstable_entry_to_json(): Use the DNSDB API rdata format

In order to make dnstable_entry_to_json() generate the same format that
the DNSDB API uses:

 - Also output the count, time_first, and time_last fields for rdata
   entries.

 - Format the rrtype as a mnemonic rather than a bare integer.
dnstable_entry_to_json(): Fix mismatch between type of e->rrtype and the format string
Merge branch 'branches/dnsdb-rdata-json' into next