feat: add external limiter support

[colinskow]

This pull request allows Bee-Queue to integrate with any non-blocking external limiter library to throttle jobs. Recently I have been using Bottleneck to force jobs to wait until they are ready to run, but that ties up my concurrency and sometimes nothing runs. This is a much better solution!

PR includes:

• 100% test coverage on new functionality
• Detailed documentation
• Updated Typescript types

Queue.process takes a function as an optional second argument, which if provided will be run just prior to executing each job. This allows you to query an external limiter and optionally reschedule the job if it is not cleared to run. The limiter query function helps the queue rapidly scan for jobs which are ready to run rather than tying up your concurrency with waiting jobs.

The limiter query function can use a callback or return a promise. The promise should resolve to an object which at minimum contains a boolean ready parameter, true if the job is ready to run.

limiterQueue.process(
  async (job) => {
    console.log(`Processing job ${job.id} after ${job.data.tries} tries`);
    // Do some work
  },
  async (job) => {
    const tries = job.data.tries + 1;
    const data = { ...job.data, tries };
    // You can plug in any non-blocking external rate limiter here
    const result = await limiter.check();
    if (result.ok) {
      return {
        ready: true,
        // (optional) this will replace the existing job data
        data
      }
    } else {
      return {
        ready: false,
        // (optional) this will replace the existing job data
        data,
        // (optional) reschedule the job to try again at this timestamp
        // if 0 or not specified, the job goes to the end of the waiting queue
        delayUntil: Date.now() + result.msUntilReady
      }
    }
  }
);

Additionally if the limiter check promise rejects or passes an error to the callback the job will be immediately failed with no retry logic.

[coveralls]

Coverage remained the same at 100.0% when pulling edc69f4 on colinskow:limiter into b9ffac9 on bee-queue:master.