Remove reliance on StoredState in relation library

[mthaddon]

The current relation library for redis relies on StoredState (per this code you pass it into the RedisRequires constructor).

The juju documentation on StoredState says "charm authors should avoid state when they can". It would be great if the library could be updated to not rely on StoredState.

[github-actions]

https://warthogs.atlassian.net/browse/DPE-2220

[mthaddon]

We've just seen a problem related to this in 3 of 8 production discourse instances after k8s workers were resized in a cluster. The units had the wrong IP in stored state even though the information was correct in the relation databag. We worked around it by entering a juju debug-code session and manually updating stored state. It would be much better if the relation library just inspected relation data directly.