Integrated job queue

This closes #404 (closed).

The API exposed for jobs in pubsweet-server matches pg-boss's API almost exactly. The only difference comes from starting and connection to a queue.

When the pubsweet-server starts, we will automatically call startQueue(), which calls pg-boss's start(). This:

1. sets up the PostgreSQL database (e.g. adds a job table, migrates to latest pg-boss schema, etc.) to support job queues.
2. is a way for pg-boss to keep an eye on jobs, and manages storage (archival, deletion, etc.).

All other job queue users should use the exported connectToJobQueue:

const { jobs: { connectToJobQueue } } = require('pubsweet-server')

const jobQueue = await connectToJobQueue()

After this, the API is exactly the same as pg-boss's: https://github.com/timgit/pg-boss

Check out the associated tests for examples of processing a job 'in-line' (https://gitlab.coko.foundation/pubsweet/pubsweet/blob/jobs/packages/server/test/jobs/jobs_test.js) or in a different process (https://gitlab.coko.foundation/pubsweet/pubsweet/blob/jobs/packages/server/test/jobs/jobs_different_process_test.js).

For completeness, one short, complete example (in a server component context) would be:

const { jobs: { connectToJobQueue } } = require('pubsweet-server')

const emailHandler = async job => {
  return sendEmail(job.data)
}

const jobQueue = await connectToJobQueue()
const queueName = 'email-queue'

// Add job to the queue
await jobQueue.publish(queueName, { 
  subject: 'Hello!', 
  to: 'some@example.com', 
  content: 'Hi again!' 
})

// Subscribe to the job queue with an async handler
await jobQueue.subscribe(queueName, emailHandler)

// Be notified on job completion with job result
jobQueue.onComplete(queueName, job => {
  console.log('Email sent to: ', job.request.data.email)
  console.log('Email job returned: ', job.response.data)
})

Containers

While it is easy to create a docker container that processes jobs (equally easy as processing from a separate process, as shown in this test: https://gitlab.coko.foundation/pubsweet/pubsweet/blob/jobs/packages/server/test/jobs/jobs_different_process_test.js), it's not easy to sensibly manage these docker containers as a part of PubSweet's functionality. As such, I've decided to keep managing docker containers out of the scope of this MR:

1. it makes the MR quite complex and testing this the integration with Docker is not trivial
2. it's not clear this integration is needed, as using docker-compose, as we currently do, is quite powerful
3. this MR provides significant and much needed utility as it is, and should land as soon as possible

We'll add Docker-based job runner examples (starting the xsweet docx to html processing in editoria), so it will be easy to implement a similar thing on your own.

That's about it! Let me know what you think :)