Unblock reconciliation during synthesis

[jveski]

Resolves #229.

Since Eno only currently retains two syntheses, it isn't possible to (correctly) reconcile resources while synthesis is in-progress. So configuration might drift in cases where synthesis is slow.

It isn't a huge problem currently since synthesis is fast, but keeping a third synthesis ends up significantly simplifying the synthesis controller, which I'm working on property-based tests and refactoring for currently.

This change adds an "in flight" synthesis that is swapped into the existing "current" slot when synthesis is complete. I tried not to touch unrelated code - the diff mostly just swaps synthesis references, with a few exceptions (I'll leave PR comments to clarify).

Backcompat

• Compositions with CurrentSynthesis.Synthesized != nil are unaffected - this is still the terminal state
• Compositions with CurrentSynthesis == nil are unaffected since the new code will be used for synthesis
• Compositions with CurrentSynthesis != nil && CurrentSynthesis.Synthesized == nil (a.k.a. in-flight synthesis) will converge eventually because the scheduling controller considers (CurrentSynthesis.Synthesized == nil) == (CurrentSynthesis == nil)