Add optional skew to interval

[phillbaker]

This adds a skew parameter to be used with intervals whose required time precision is lower, reducing the simultaneous load on workers when multiple tasks are kicked off at the same time.

Example usage:

resources:
- name: 5-ish-min
  type: time
  source:
    interval: 5m
    skew: 1m

jobs:
- name: something-every-4-to-6m
  plan:
  - get: 5-ish-min
    trigger: true
  - task: something
    config: # ...

[vito]

Thanks for the PR!

I don't think this will help much though. When there are 10 jobs all fetching from the same 'skewed' resource (as the skew would only affect the version discovery, not the resulting scheduling), you'll still end up with a stampede.

This may be better addressed at the root of the concern, which is scheduling workloads across jobs.

There's also serial_groups, though that will eliminate all parallelism.

Related: concourse/concourse#675

[phillbaker]

Thanks for taking a look. We were on an old version of concourse and in the 3.0 refactor to reuse containers this does become useless.