Do not use raw pointers to index CARP CachePeers (#1381)

Do not use raw pointers to index CARP CachePeers (#1381)

Simplified and improved code safety by using CbcPointers instead.

Also fixed mgr:carp Cache Manager reports to detail relevant
cache_peers instead of all cache_peers. When mgr:carp report was added
in 2000 commit 8ee9b49, Squid did not index (or even distinguish!)
CARP cache_peers, and the reporting loop naturally iterated through all
cache_peers. 2002 commit b399543 added identification and indexing of
CARP peers but forgot to adjust the reporting loop.

Very similar changes will be applied to userhash and sourcehash
cache_peers. 2008 commit 63104e2 simply copied problematic CARP code to
add userhash and sourcehash cache_peer support. This change adds a few
reusable types with those upcoming improvements in mind.