compressed data msg marker

[Zetanova]

It is common to exclude already compressed data from a secound pass in wire and/or persistent storage.

One of the oldest example would be the exlusion of http gzip comp. For jpeg/PNG files.

It would be a good idea to include a well-known msg header to mark a msg data as compressed.

Even if it is not instantly used by the server components, it will have its usage in the feature.

One of main benefit of handle compressed data exclusively, beside less processing req, is that the compressed data will not poison the compression stream/dictionary

Because the marker would be also valid for encrypted data, the header should mark a message uncompressable.

Example: x-nats-flags: nocomp