Fix PeerDigest lifetime management (#1857)

Fix PeerDigest lifetime management (#1857)

This change fixes how cbdata is used for managing CachePeer::digest
lifetime. Prior to these changes, cbdata was (ab)used as a reference
counting mechanism: CachePeer::digest object could outlive CachePeer
(effectively its creator), necessitating complex "is it time to delete
this digest?" cleanup logic. Now, CachePeer is an exclusive digest owner
that no longer locks/unlocks its digest field; it just creates/deletes
the object. Digest fetching code no longer needs to cleanup the digest.

"CachePeer::digest is always valid" invariant simplifies digest fetching
code and, hopefully, reduces the probability of bugs similar to Bug 5329
fixed in minimal commit 4657405 (that promised this refactoring).