VAST changes for spatial compaction

[lava]

This contains 4 independent changes that are required by the compaction plugin.

📝 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

Review this pull request commit-by-commit.

[tobim]

My main concern is the deep copy of the partition synopsis, I don't understand why this is necessary.

[tobim]

I approve of the high-level semantics that are implemented here.

[mavam]

I want f6fc28a in my topic branch. Should I cherry-pick or is this one going to merged soon?

EDIT: looks like auto-merge is on, so only a CI thingy left.

[dominiklohmann]

I think this just needs to be adapted to FlatBuffers naming changes on master :)

[dominiklohmann]

Found some minor stuff that should be fixed before merging.