Allow configuring whether throttled jobs are put back on the queue immediately or scheduled for the future

[lavaturtle]

This adds support for setting the requeue_strategy for a job, to specify what we should do with throttled jobs. The default is :enqueue, which is the current behavior: re-add it to the end of the queue. The other option is :schedule, which schedules the job for a time in the future when we think we'll have capacity to process it.

It's also possible to set the default_requeue_strategy in the configuration, to set this behavior for all jobs that do not individually specify a requeue_strategy.

This may be relevant to Issue #36.

Unrelatedly, there's a commit in here that changes the format of some require_relative lines, to comply with the new Rubocop rule Style/RedundantCurrentDirectoryInPath. I don't feel strongly about this commit; I only added it so that rubocop would pass.