Summary: We should not use IterKey::SetKey with copy = false except if we are pinning the iterator thru it's life time, otherwise we may release the temporarily pinned blocks and in this case the IterKey will be pointing to freed memory

Test Plan: added a new test

Reviewers: sdong, andrewkr

Reviewed By: andrewkr

Subscribers: andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D57561