primary key still) instead of using the uuid of the slice. We do this
so that we can easily cache slice credentials for different users (or
multiple credentials of any kind).

Add IsExpired() and SameCerts() tests.

Add DeleteForTarget() which remove asny cached credentials for a
target object (such as a slice), as when a slice is expired or
deleted.

Add sanity check to watch for duplicate credentials (same target and
owner). Never supposed to happen.