Cache: metadata should not change for a Found

[cceckman-at-fastly]

Found via testing in Compute-

Viceroy is written in such a way that metadata associated with a Found can change; namely, Found can get a known length after the fact (see poll_known_length in the test suite).

This can't actually happen in Compute -- the same poll_known_length function either immediately gets a length, or fails to ever get a length.

The Viceroy implementation should work the same way - metadata, including length, is locked-in at lookup time.