A signed "keep" edge for GC/indexing purposes.  Expressed a user's
intent to keep an object.

This is not the only way to keep an object alive for the purposes of
GC.  Permanodes (signed by a user) are also part of that user's roots,
and anything they reference (including blobs via "become" claims on
those permanodes)

This is just the most explicit way when you're not modeling the data
with permanodes.

{"camliVersion": 1,
 "camliType": "keep",
 "target": "digalg-blobref-of-thing-to-keep",
<REQUIRED-JSON-SIGNATURE>}