Add Traffic Ops Batch Endpoint

[rob05c]

This is a Proof-of-Concept. I just needed to prove to myself that it's reasonable, it still needs the details worked out, docs, tests, etc. I'm making a Draft PR in case anyone else is interested in it.

It allows users to make a single atomic request, such as

POST https://fqdn.example/api/5.0/batch/ -d '[{
"method": "GET",
"path": "/api/5.0/ping"
}, {
"method": "GET",
"path": "/api/5.0/cdns"
}]' 

And get a response, with each member of the batch executed with sequential consistency (that is, GETs may be executed in parallel, but everything will appear to have been executed sequentially, e.g. GET-POST-GET will have the POST changes in the second GET but not the first), like:

[ { "code": 200,
    "headers": {
      "Access-Control-Allow-Headers": [ "Origin, Content-Type" ],
      "Content-Type": [ "application/json" ] },
    "body": { "ping": "pong" }
  }, {
    "code": 200,
    "headers": {
      "Access-Control-Allow-Headers": [ "Origin, Content-Type" ],
      "Content-Type": [ "application/json" ], },
    "body": { "response": [ {
          "id": 42,
          "lastUpdated": "2001-01-01 12:34:56+00",
          "name": "ALL"
        }, {
          "domainName": "cdn.example.net",
          "id": 24,
          "lastUpdated": "2001-01-01 12:34:56+00",
          "name": "my-cdn"
          } ] } } ]

The main goal here is atomicity, not convenience. By allowing users to execute multiple requests atomically, we can make it safe to do things that aren't safe today. For example, changing a Delivery Service, changing it's Profile, and seven different Parameters, all at once. This doesn't completely solve the Snapshot-Queue Automation a.k.a. "Kill the Chicken" problem, but it addresses a large subset of it, in a much smaller feature than a complete solution.

It's unlikely I'll have time to finish this. But anyone else is welcome to fork and make a real PR from it.

Which Traffic Control components are affected by this PR?

• Traffic Ops

What is the best way to verify this PR?

If this is a bugfix, which Traffic Control versions contained the bug?

PR submission checklist

• This PR has tests
• This PR has documentation
• This PR has a CHANGELOG.md entry
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)