[POC] [5.6] Allow chaining multiple queue jobs

[JosephSilber]

The code here is a (fully functional) proof of concept, so it's not fully fleshed out and has no tests.

---

To test, run php artisan queue:chain-table to publish the queue chain table migration. Then run php artisan migrate to actually create the table. Now you're ready to start chaining like a boss 😎

---

Allows you to chain a job after multiple jobs run in parallel. For example:

dispatch([new SomeJob(1), new SomeJob(2), new SomeJob(3)])->chain(
    new RunsAfterOthersHaveCompleted(),
);

All 3 SomeJobs will run in parallel. When they're done, the final one will run.

---

You can construct these chains as complex as you need them to be. At each step, you can have as many parallel jobs as needed. Here's another example:

dispatch([new SomeJob(1), new SomeJob(2)])->chain(
    [new SomeOtherJob(1), new SomeOtherJob(2)],
    [new YetAnotherJob(1), new YetAnotherJob(2)]
);

The SomeJobs will run in parallel. When done, the SomeOtherJobs will run in parallel. Finally, all YetAnotherJobs will run in parallel.

If you're familiar with JS promises, the above is similar to this:

Promise.all([someJob(1), someJob(2)])
       .then(() => Promise.all([someOtherJob(1), someOtherJob(2)]))
       .then(() => Promise.all([yetAnotherJob(1), yetAnotherJob(2)]));

---

This PR is a resubmission of #19515, with the race condition fixed.

[Miguel-Serejo]

dispatch([new SomeJob(1), new SomeJob(2)])->chain(
   [new SomeOtherJob(1), new SomeOtherJob(2)],
   [new YetAnotherJob(1), new YetAnotherJob(2)]
);

Would it make more sense to chain calls to ->chain()?

dispatch([new SomeJob(1), new SomeJob(2)])->chain(
    [new SomeOtherJob(1), new SomeOtherJob(2)])->chain(
    [new YetAnotherJob(1), new YetAnotherJob(2)]
);

[Artistan]

chaining calls to ->chain() would be more synonymous with the JS Promise example that you referenced....

[taylorotwell]

Holding off on this for now.