reftable: make reftable_record a tagged union

This reduces the amount of glue code, because we don't need a void
pointer or vtable within the structure.

The only snag is that reftable_index_record contain a strbuf, so it
cannot be zero-initialized. To address this, use reftable_new_record()
to return fresh instance, given a record type. Since
reftable_new_record() doesn't cause heap allocation anymore, it should
be balanced with reftable_record_release() rather than
reftable_record_destroy().

Thanks to Peff for the suggestion.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

reftable/block.c

@@ -421,7 +421,7 @@ int block_reader_seek(struct block_reader *br, struct block_iter *it,
 done:
 	strbuf_release(&key);
 	strbuf_release(&next.last_key);
-	reftable_record_destroy(&rec);
+	reftable_record_release(&rec);
 
 	return err;
 }