Support pipeline cancellation with Go's native Context

[ccremer]

Summary

• Replaces the pipeline.Context type with context.Context throughout the code. To pass data around, use context.WithValue instead
• Pipelines can now be canceled. In case of cancellation, now steps are scheduled for execution, however each step has to listen for the cancellation event themselves.
• Support canceling parallel.NewFanoutStep with changed PipelineSupplier usage. See the example in godoc on how to properly configure this.
• Support canceling parallel.NewWorkerPoolStep with changed PipelineSupplier usage. See the example in godoc on how to properly configure this.

Checklist

• Categorize the PR by setting a good title and adding one of the labels:
  fix, enhancement, documentation, change, breaking, dependency
  as they show up in the changelog
• Update documentation.
• Update tests.