Add retry logic to workerutils

Created by: mrnugget

As we discussed in Slack: it would be great to have retry-functionality in the workerutils package.

What would that look like? Handlers could return special errors that are "retryable" and if they are, the worker enqueues the job again.

Sketch:

err := handler.process(ctx, tx, record)
if err != nil && errcode.IsRetryable(err) {
  EnqueueRetry(ctx, record)
}