When sending cached_dir_t objects compressed with zlib, do not claim zstd compression.
Right now our code in handle_status_vote sends cached_dir_t objects in their deflated variant when compression is negotiated... but it claims to have compressed them with zstd when both sides support that.
This has been a bit of a heisenbug, since it only happens when we're downloading votes, which doesn't happen with the usual timing on chutney.
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