This completes the data/functionality migration for multiple DNS writers.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=163835077
Note that even though the nomulus command line tool now supports multiple
DNS writers for all subcommands, this still won't work quite yet because
the DNS task queue format migration from [] is still in progress.
After next week's push that migration will be complete and we can remove
the final restriction against only having one DNS writer per TLD.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=162490399
This is written in such a way that it can safely handle task items in the
old format so long as the DNS writer to use for the given TLD is unambiguous
(which it is for now, until we allow multiple DNS writers to be configured).
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=162293412
This is one of several CLs in a sequence for allowing per-TLD DNS
implementations.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=129445641