Do not forward HTTP requests to dead idle peers.

Do not forward HTTP requests to dead idle peers.

Squid does not forward HTTP transactions to dead peers except when a
dead peer was idle for some time (ten peer connect timeouts or longer).
When the idle peer is still dead, this exception leads to transaction
delays (at best) or client disconnects/errors (at worst), depending on
Squid and client configurations/state. I am removing this exception.

The "use dead idle peer" heuristic was introduced as a small part of a
much bigger bug #14 fix (trunk r6631). AFAICT, the stated goal of the
feature was speeding up failure recovery: The heuristic may result in
HTTP transactions sent to a previously dead (but now alive) idle peer
earlier, before the peer is proven to be alive (using peer revival
mechanisms such as TCP probes). However, the negative side effects of
this heuristic outweigh its accidental benefits. If somebody needs Squid
to detect revived idle peers earlier, they need to add a different
probing mechanism that does not jeopardize HTTP transactions.

Nobody has spoken in defense of this feature on Squid mailing lists:
http://lists.squid-cache.org/pipermail/squid-users/2017-March/014785.html
http://lists.squid-cache.org/pipermail/squid-dev/2017-March/008308.html

The removed functionality was not used to detect revived peers. All peer
revival mechanisms (such as TCP probes) remain intact.