Fix shutdown with importers running

[dominiklohmann]

📔 Description

This fixes a recently introduced bug—the shutdown was not working while imports were still running.

📝 Checklist

• All user-facing changes have changelog entries.
• The changes are reflected on docs.tenzir.com/vast, if necessary.
• The PR description contains instructions for the reviewer, if necessary.

🎯 Review Instructions

n/t

[lava]

Changes look good to me, a regression test would be nice but as the comment already explains this is a much bigger issue we still need to solve.