Unify usage of Redis across the codebase

[Oaphi]

Currently, we have 2 separate and differently configured Redis uses:

• via the NamespacedEnvCache cache store (which uses connection pools under the hood, properly handles thread safety, etc). Calling code usually looks like Rails.cache.*;
• directly in thread-specific RequestContext (with no pooling, QoL methods, etc). Calling code looks like RequestContext.redis.*;

We should fully switch to the former version:

• for consistency;
• to remove a redundant connection;
• to reduce maintenance burden;
• to enable connection pooling where we currently use one per thread (the latter use);