sealing: Use only non-assigned deals when selecting snap sectors

[magik6k]

Related Issues

Likely fixes #10914
Thanks to added logging in #10915

Proposed Changes

The sealing pipeline keeps pending piece references for a while after deals are assigned/added to sectors. When selecting what sector to upgrade, we calculate a list which says "at Y epoch there are X deals waiting to be inserted into a sector". That logic only cares about deals/pieces which weren't added/assigned to any sector, which we didn't check, causing us to potentially select sectors which couldn't accept any deals, which would cause the pipeline to fill up with waiting sectors, just waiting for a timeout..

Additional Info

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