fix: BulkWriter flush logic

[tom-andersen]

Should fix: #1772

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[milaGGL]

LGTM. Thank you Tom.

[thewildllama]

I've tested this a few times, and it still hangs occasionally. I'd have to add more logging to figure out where things went off the rails... :(

[bdaz]

@tom-andersen Any chance there is a similar issue in the JS version of BulkWriter? I am seeing the same behavior, where all writes actually succeed, but flush() (or close()) hang.