Replace Python string formatting with calls to native code

Make the Python entry.to_json() and entry.to_text() methods thin
wrappers for dnstable_entry_to_json() and dnstable_entry_to_text().

In a libdnstable with the new dnstable_entry_set_iszone() function, the
native text and JSON format functions produce equivalent output to what
we were previously producing with the Python code, and the native
functions are much faster, of course.