Do not blame cache_peer for 4xx CONNECT responses (#1166)

Do not blame cache_peer for 4xx CONNECT responses (#1166)

To avoid using a problematic cache_peer, Squid penalizes cache_peers for
a variety of problems. However, a problem like an HTTP 403 Forbidden
response may be caused by the client or other factors unrelated to the
cache_peer that sent the response to Squid. In those cases, the
cache_peer is not at fault and should not be marked dead, even after
many such responses.

This change stops blaming cache_peers for HTTP 4xx outcomes of Squid
CONNECT requests. Currently, such outcomes only happen when a cache_peer
responds with a 4xx reply, but the new code also treats Squid-generated
4xx error responses (while trying to establish a connection to the
cache_peer) the same way.

This hard-coded logic covers known use cases. If different use cases
surface, we can make Squid behavior configurable.

Co-authored-by: Amos Jeffries <squid3@treenet.co.nz>