Style/SpaceBeforeBlockBraces
Style/SpaceInsideBlockBraces
Style/SpaceInsideHashLiteralBraces

Fix all violations in the repository.

The current code base is not uniform. After some discussion,
we have chosen to go with double quotes by default.

* Use separate stream handler builders for easy override and testing.
* Fix worker pool execution that was silently failing since it only
  expected connection receivers.

Sparked by code in #24162.

* Introduce a connection coder responsible for encoding Cable messages
  as WebSocket messages, defaulting to `ActiveSupport::JSON` and duck-
  typing to any object responding to `#encode` and `#decode`.
* Consolidate encoding responsibility to the connection. No longer
  explicitly JSON-encode from channels or other sources. Pass Cable
  messages as Hashes to `#transmit` and rely on it to encode.
* Introduce stream encoders responsible for decoding pubsub messages.
  Preserve the currently raw encoding, but make it easy to use JSON.
  Same duck type as the connection encoder.
* Revert recent data normalization/quoting (#23649) which treated
  `identifier` and `data` values as nested JSON objects rather than as
  opaque JSON-encoded strings. That dealt us an awkward hand where we'd
  decode JSON strings… or not, but always encode as JSON. Embedding
  JSON object values directly is preferably, no extra JSON encoding,
  but that should be a purposeful protocol version change rather than
  ambiguously, inadvertently supporting multiple message formats.

Fixes #22675

Allow channel identifiers and also data with no backslahes/escaping to be accepted by
the subscription storer.

This reverts commit d0393fcc, reversing
changes made to 3b7ccadf.

This removes 8 runtime gem dependencies from Rails:

```
Using hitimes 1.2.3
Using timers 4.1.1
Using celluloid-essentials 0.20.5
Using celluloid-extras 0.20.5
Using celluloid-fsm 0.20.5
Using celluloid-pool 0.20.5
Using celluloid-supervision 0.20.5
Using celluloid 0.17.2
```

Without this, it's very easy to send messages over a subscription even before the
redis pubsub has been fully initialized.

Now we delay calling the subscription#connected method on the client side until we
receive a subscription confirmation message from the server.

* Move specific requires close to where they're needed.
* Use the private active_support/rails dep to wrap up common needs
  like eager autoload and module delegation.
* Use a single Rails engine rather than an engine and a railtie.
* Prefer `AS::JSON.encode` to `Object#to_json`.

The run_in_eventmachine test helper method is setting a 0.1 second timer to stop the event machine loop. This causes each test that requires an event machine loop to wait for 0.1 second regardless of how long the test takes to process.

This changes that to call EM.run_deferred_callbacks, which immediatly process pending actions in the event loop and then is able to exit the event loop without doing any waiting.

Before this change, running tests produced

    Finished in 2.957857s, 15.8899 runs/s, 27.7228 assertions/s.

After, the tests get

    Finished in 0.065942s, 712.7514 runs/s, 1243.5237 assertions/s.