Clarify lifecycle of Constraint#predicate Predicate

[findepi]

Constraint#predicate is an opaque Predicate that internally holds to an evaluator.
this is passed to ConnectorMetadata#applyFilter

Question:

• is connector allowed to hold onto this predicate after ConnectorMetadata#applyFilter returns?

Regardless of the answer, since this is a subtle aspect of the SPI, we should document that in the javadoc.

[martint]

No, it's not allowed to hold on to it. It's meant to be used only for the duration of the API call.

[findepi]

That would be my intuition as well, but i didn't know the intentions.

1. we should document
2. we can optionally enforce that, by "destroying" the evaluator after the call returns