This PR is the Rails companion for the Turbo changes to add page
refreshes.

```ruby
turbo_refreshes_with scroll method: :morph, scroll: :preserve
```

This adds new Active Record helpers to broadcast page refreshes from
models:

```ruby
class Board
  broadcast_refreshes
end
```

This works great in hierarchical structures, where child record touch
parent records automatically to invalidate cache:

```ruby
class Column
  belongs_to :board, touch: true # +Board+ will trigger a page refresh
  on column changes
end
```

You can also specify the streamable declaratively:

```ruby
class Column
  belongs_to :board
  broadcast_refreshes_to :board
end
```

There are also instance-level companion methods to broadcast page
refreshes:

- `broadcast_refresh_later`
- `broadcast_refresh_later_to(*streamables)`

This PR introduces a new mechanism to suppress broadcasting of turbo
treams for arbitrary blocks of code:

```ruby
Recording.suppressing_turbo_broadcasts do
...
end
```

When broadcasting page refreshes, the system will automatically debounce
multiple calls in a row to only broadcast the last one. This is meant
for scenarios where you process records in mass. Because of the nature
of such signals, it makes no sense to broadcast them repeatedly and
individually.

…/turbo

A page refresh stream signals the need to reload the page. It's a global action
that makes sense to aggregate when multiple signals are generated in a short period
of time for a given streamable.

This implementation is based on creating a thread-level debouncer associated to
the set of streamables. The debouncer is implemented using concurrent-ruby's
scheduled tasks.

…/turbo

This can happen when the Debouncer has finished its work and we clear it
from the current thread. The next call to refresh_debouncer_for creates
a new Debouncer, but it doesn't have a scheduled_task yet.

We only use the wait method in tests, to ensure that the debounces has
finished its work. But if the scheduled_task is nil, we know that the
debouncer has already finished its work.