updata: allow specifying a start time for waves

[tjkirch]

Previously, wave start times would always be relative to when you ran updata,
meaning you'd have to hold off on running updata until you were ready to start
a release process. This lets you (optionally) pick any start time for the
start of the waves, and the given start_after offsets will be relative to that
time.

To keep the code and the deployment process simple, this also requires
specifying relative offsets in wave definition files, rather than absolute
times; we only used relative offsets anyway so that the files were reusable and
it was easier to understand the delays between waves, but this codifies that
behavior.

---

This caused pain in the 0.3.3 release, requiring rework and a changed deployment plan, so I wanted to fix it before the next release.

Testing done:

Start with no waves:

$ jq '.updates[].waves' manifest.json
{}

Add waves with no start-at argument, as we have before:

$ cargo run --bin updata -- set-waves manifest.json --arch x86_64 --version 0.3.3 --variant aws-k8s-1.15 --wave-file ../waves/accelerated-waves.toml

We see the waves are relative to when I ran the command, as before.

$ jq '.updates[].waves' manifest.json
{
  "0": "2020-05-14T23:06:11.364246960Z",
  "61": "2020-05-15T02:06:11.364246960Z",
  "245": "2020-05-15T06:06:11.364246960Z",
  "1024": "2020-05-15T22:06:11.364246960Z"
}

$ date -u
Thu May 14 22:06:18 UTC 2020

Now run with a start-at next month, and we see the waves start next month as expected:

$ cargo run --bin updata -- set-waves manifest.json --arch x86_64 --version 0.3.3 --variant aws-k8s-1.15 --wave-file ../waves/accelerated-waves.toml --start-at '2020-06-01T00:00:00Z'

$ jq '.updates[].waves' manifest.json
{
  "0": "2020-06-01T01:00:00Z",
  "61": "2020-06-01T04:00:00Z",
  "245": "2020-06-01T08:00:00Z",
  "1024": "2020-06-02T00:00:00Z"
}

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[zmrow]

🎧

Nice and clean. :)