Delivery Service Snapshots

[Robert Butts <ro...@apache.org>]

I have a WIP PR for Delivery Service Snapshots, using the timestamp
versioning plan:
https://github.com/apache/trafficcontrol/pull/3175

It follows
https://cwiki.apache.org/confluence/display/TC/Traffic+Control++Self+Service+Proposal+for+Change+Integrity

With one major exception: it creates new "_snapshot" tables mirroring the
"real" tables, which are copied into on snapshot. This is because Perl
touches these tables, and the Perl can't be easily fixed.

We've had a hard time finding the resources to rewrite all of Perl. If we
want, this gives us a way forward to Self Service / solving the
Snapshot-Queue problem / deploying changes to a single DS, without
affecting any other DS or data. Then, when Perl is completely rewritten, we
can simply rename the "_snapshot" tables in a migration. See the Github PR
for more details.

It's still WIP; but it essentially works, if anyone wants to pull and try
it out.

But, I wanted to get feedback. Is anyone opposed to this idea? Creating
"_snapshot" tables for DS Snapshots now, and renaming them later? Opinions?
Suggestions?