changefeedccl: Correctly account memory when flushing compressed files

[miretskiy]

Cloudstorage sinks incorrectly account for memory when flushing compressed
files to cloud storage.
When closing "codec" we may write additional bytes to memory buffer, and those bytes must be correctly accounted for.

[blathers-crl]

Hi @miretskiy, please add branch-* labels to identify which branch(es) this release-blocker affects.

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