Stop leaking PeerDigests on reconfiguration (#1371)

Stop leaking PeerDigests on reconfiguration (#1371)

peerDigestCreate() cbdata-locked the digest twice. The second lock was a
"self lock" -- a lock without storing the locked pointer somewhere. That
self lock was introduced (with an XXX) in 2002 commit fa80a8e. It did
not have the corresponding unlock, and Squid leaked the digest object.
That leak became conditional in 2018 commit b56b37c: Since then, Squid
was missing one digest unlock only when the CachePeer object was gone
before a peerDigestDestroy() call, as happens during reconfiguration.

Also removed a pair of excessive cbdata locks/unlocks (that triggered
this leak): Long-term users should lock when they start storing a
pointer to the cbdata-protected object and unlock when they stop.

Also converted PeerDigest::peer to CbcPointer, addressing a TODO.

Also fixed subtle problems with detecting gone CachePeer and gone
PeerDigest objects in peerDigestFetchedEnough().

Also removed stale peerNoteDigestGone() declaration.