Decouple Redis from AnyCable

[palkan]

Context

AnyCable no longer depends on Redis in production, so shouldn't we.

Changes

• Decoupled Redis from AnyCable.broadcast_adapter (and removed all usages of the broadcast adapter)
• Refactored Redis usage to be connection pool-friendly (i.e., #with_redis { |conn| ... } instead of #redis). Although we do not use a connection pool by default, some users might want to go that way.

[Envek]

In general I'm all for it 👍

Below are some nitpicks and suggestions to discuss (all are optional)

[palkan]

@Envek Thanks!