[NEW] Enable using a connection while waiting for a blocking command about valkey HOT 3 OPEN

I like the opt-in for a possible +ASYNC reply to async commands, with the actual reply delivered later using Push. It solves the problem for clients. They can hide this handling from the end user. I have thought about this and I can't find any real problem with it. I would accept a PR.

Multiplexing. It solves more problems than just blocking commands. I don't like the suggested solution in that issue though. Instead we can support RESP over QUIC, with optional linking with a QUIC library, such as MsQuic. QUIC is unusual now, but it will be more common in the future.

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Can't you avoid blocking commands by using some form of the client-side-caching or key-space notifications?

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@mzimbres can you? for every blocking command?

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