Does Factor.state::Recipestate need a state label?

[Affie]

Open questions from #1179
Why WIP:

• state::Recipestate might need a state label to allow multiple solves to run against the graph concurrently.
• You cannot build 2 trees at the same time.

[Affie]

Also related, does Factor.cache need separate state labels or are they always copied. The cache might need to move into factor.state?