Instance methods can be defined in the module itself

Adds changelog headers for beta3 release

[ci skip]

This PR includes two changes for 2.0.0:

- Breaking API change around `async.perform` --> `perform_async`
- New addition of `perform_in`, which now allows end users of the
  adapter to use the `enqueued_at` public API method.

[ci skip]

During the `5.0.0.beta1` release, the CHANGELOGs got an entry like the
following:

```
* No changes.
```

It is kinda confusing as there are indeed changes after it. Not a
biggie, just a small pass over the CHANGELOGs.

[ci skip]

🎉 🍻

According to pr #22443 in the guides there's always a dollar sign before every command, so why is in the main README a `$` and in every submodule a `%`?

Just eye candy..

On most filesystems file ctime is limited to 1 second granularity, which means that on
faster computers multiple simple jobs (for instance dummy TestJob) can finish within the
same second.

The execution order test in ActiveJob integration tests relies on multiple TestJobs
writing files then comparing the ctime. As a result integration tests would sometimes
fail as the ctime of the files written by these TestJobs could have coincidental ctimes
making the comparison for job order fail.

This commit adds a far more precise execution time (to the extent that the Ruby Time
class allows) to the file created by TestJob, and updates the execution order assertion
to use it, removing the race condition.

we are using `@monitor = Monitor.new` that inherit from Monitor class, we leave behind this commit https://github.com/rails/rails/commit/cbfc8b36

[CI skip]

The INITIAL_WAIT constant has moved to the Sidekiq::Poller class but
rather than setting the constant directly we can override it via the
`:poll_interval_average` option.

This was causing random build failures because the test was waiting
for 10 seconds for the job to execute but the initial wait was a
random value between 10 and 15 seconds.

If db/schema.rb doesn't exist then we get warnings from the dummy Rails
application so run it for all adapters even if they're not using the
database to store jobs.

The thread_safe gem is being deprecated and all its code has been merged
into the concurrent-ruby gem. The new class, Concurrent::Map, is exactly
the same as its predecessor except for fixes to two bugs discovered
during the merge.

Refactor arguments logging method for Active Job

Currently the log returned by running ActiveJob tests is filled with:

> DEPRECATION WARNING: Returning `false` in a callback will not implicitly halt a callback chain in the next release of Rails. To explicitly halt a callback chain, please use `throw :abort` instead.

For instance, see https://travis-ci.org/rails/rails/builds/77978273

This happens because some setup and teardown methods [like these one](https://github.com/rails/rails/blob/master/activejob/test/cases/async_job_test.rb#L10-L17)
invoke other methods like `perform_asynchronously!` that can return `false`, but
not with the intention of halting the process if they do.

In my opinion, these deprecation warnings can be silenced to have
the log result cleaner (especially when browsing for errors).