regression: dataloss as a result of prematurely "finish"ed arrows

[de-sh]

This is a regression happening after #1129 was merged.

Report: https://gist.github.com/de-sh/6648759b53bce7a7e21efd3319e100bb

ref: test that showcases this effect in practice

Possible Fix

Make flush intelligent enough to choose not to finish arrow from the current minute-slot.

[de-sh]

The fix has been worked on and will be introduced as a PR post merging of #1238

de-sh/parseable@disk-writer...de-sh:parseable:is_current