Add a bin/jobs binstub to run the supervisor more easily and remove the async mode

[rosa]

This also fixes that the supervisor couldn't be started in async mode unless using the Puma plugin This completely removes the async mode, as we've decided to make this straightforward and simpler and offer just a single way to run it.

This PR also includes other minor issues in the installation: avoiding overwriting an existing config/solid_queue.yml file and accepting a --database parameter to specify a separate Solid Queue DB to install the migrations. The new bin/jobs binstub is copied over as part of the installation.

[floehopper]

Is the new bin/jobs bin stub equivalent to bundle exec rake solid_queue:start? Is either one preferred over the other?

[rosa]

Great question! It's mostly equivalent, witih a few differences. It allows passing a --config_file option with a different location for the workers and dispatchers configuration (with the Rake task you need to set an environment variable, SOLID_QUEUE_CONFIG, if you wanted to change that). It also behaves a bit differently with respect to eager loading, I believe, because Rake tasks ignore config.eager_load = true, there's an additional setting config.rake_eager_load to get this enabled, but I think many people might not have it enabled: rails/rails#28209 (comment)

I'd say bin/jobs is preferred, to be consistent with other Rails commands/tasks that have their own binstub like this.

[floehopper]

@rosa Thanks for explaining!