`prune` to return whatever it has in the case of incomplete read by default

[bienur]

Instead of returning nothing when the read is incomplete, a better default for prune query maybe to return whatever that it has gotten. Also surface isComplete from prune as part of the return for Cache.extract (if prune query is provided as arg) so that client of Hermes can perform follow-up if they so choose.

Bonus: make the behavior configurable ^^