We're no longer doing our work in the EM event loop, so errors are quite
unlikely, and if they do occur, they're not really our responsibility to
handle.

(as adapted to use concurrent-ruby / nio4r instead of eventmachine)

Just like their ActiveJob equivalents, these only work within the
current process.

Fixes #23193

This was throwing `There was an exception - NameError(undefined local variable or method `hi_redis_conn' for #<ActionCable::SubscriptionAdapter::Redis:0x007fb1449e2b70>)` on unsubscribe.

- adapter -> pubsub (re)rename internally
- Change variable names to match method names
- Add EventMachine `~> 1.0` as a runtime dependency of ActionCable
- Refactor dependency loading for adapters

- Escape the channel name when subscribing in PG
- Refactor popping the queue to make it easier to read

We now only create one adapter instance for the server, so it can hold
the listener. This in turn allows the listener to get the PG connection
from the adapter, which will be a good place to allow more flexible
configuration.

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

Found this small omission while reviewing the ActionCable docs.
[Dave Moore]

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
```

Added to aid in developing Action Cable, but isn't neccessary for production operation.

🎉 🍻

```
# before
App.appearance = App.cable.subscriptions.create "AppearanceChannel",
  connected: ->
    # Called when the subscription is ready for use on the server

  disconnected: ->
    # Called when the subscription has been terminated by the server

  received: (data) ->
    # Called when there's incoming data on the websocket for this channel

  appear: ->
    @perform 'appear'
  away: ->
    @perform 'away'
```

```
# after
App.appearance = App.cable.subscriptions.create "AppearanceChannel",
  connected: ->
    # Called when the subscription is ready for use on the server

  disconnected: ->
    # Called when the subscription has been terminated by the server

  received: (data) ->
    # Called when there's incoming data on the websocket for this channel

  appear: ->
    @perform 'appear'

  away: ->
    @perform 'away'
```