refactor: prefer returning Result from Redis client when possible

[carodewig]

This PR covers a few small changes to the Redis storage layer:

• Refactor the Redis cache API to surface more errors to callers, which will allow us to return clearer results as part of response caching
• Correct the out-of-date doc comment for the Redis timeout - it claimed that the default was 2ms rather than 500ms
• Set max_command_buffer_len so that the number of inflight Redis commands is bounded. This should increase the stability of the Redis client under load, as it will return Backpressure errors rather than queueing forever

I don't believe any of these changes are user-facing so have not added a changeset.

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