Add active partition actor to unpersisted partitions on decomission

[patszt]

📝 Reviewer Checklist

Active partition actor will be inserted into unpersisted partitions on decomission.
While running export and import concurrently we received many warning messages because we couldn't receive any partition_actor that was part of a query.

Review this pull request by ensuring the following items:

• All user-facing changes have changelog entries
• User-facing changes are reflected on vast.io

[lava]

We already looked at this in a video call; the fix looks obviously correct so we will merge it and test in our testbed in the upcoming days.