Fix: Prevent routing reads to Redis slave nodes in loading state

[ofekshenawa]

Problem

When a new slave node is added to a Redis cluster, it enters a loading state while it synchronizes data from its master. If the client has ReadOnly=true enabled, it may route read operations to this loading slave node, which can result in nil values or errors being returned to the application.
closes #3222

Solution

This PR adds logic to detect when a slave node is in the loading state (by checking for LOADING errors) and temporarily excludes such nodes from the read distribution until they're ready to serve requests.

[ndyakov]

@ofekshenawa I am not sure (was not able to find information) if PING will return -LOADING errors. Let's think of a way to test this somehow? We will need to write tests for it anyway. Sure, we can mock the response and we will get the correct execution, but not sure on top of my head how to actually trigger a redis node to load the rdb for longer periods of time, so we can test. Any ideas?

[ndyakov]

Update: PING at the moment should return LOADING, but it may change. Keep that in mind. Let's think of a way to test this.