Option to disable punsubscribe call

[acjay]

RedisPubSub unconditionally runs both an UNSUBSCRIBE and a PUNSUBSCRIBE command when unsubscribing. Normally, this probably isn't an issue, but ElastiCache Serverless doesn't support PUNSUBSCRIBE, causing an exception, and preventing the rest of the cleanup from happening. It would be helpful to have an option not to run the PUNSUBSCRIBE.

[sondremare]

Another suggestion on how to avoid this problem: #669