Handle invalidate messages from topic __redis__:invalidate

[chisenberg]

This pull request handles the "invalidate" messages sent by redis when enabling CLIENT TRACKING and subscribing to topic "__redis__:invalidate" topic.
The invalidated keys are sent on the PayloadSlice field of the redis.Message struct.

[ndyakov]

@chisenberg thank you for the contribution. Would you please add tests for the code you are introducing?

[ndyakov]

@chisenberg are you using the client for a client side caching ? We don't have it inside the client for now, but would like to add it in the near future.

[ofekshenawa]

Hey @chisenberg ,
It looks like the same functionality has already been addressed in #3007.

[chisenberg]

@chisenberg are you using the client for a client side caching ? We don't have it inside the client for now, but would like to add it in the near future.

Sorry for the delay. Yes, we are using the client side caching feature on our product. It is working great on my fork but I still didn't write the unit tests that you asked.

[ndyakov]

Hello @chisenberg, There is no way for us to merge this if there are no tests. Would you mind adding tests?