Implement keyed priority queue

[daun]

Description

• Re-implement throttled queue as class with keyed Maps
• Goal: key jobs by URL to allow handing out Promises
• Replace the job queue and the preloadPromises Map to a single this.queue

Other changes

• Switch to async-await for adding the PageData promise

Checks

• The PR is submitted to the master branch
• The code was linted before pushing (npm run lint)
• All tests are passing (npm run test)
• New or updated tests are included
• The documentation was updated as required

Additional information

Closes #95