Close idle client connections associated with closed idle pinned connections.

Close idle client connections associated with closed idle pinned connections.

Squid was not monitoring idle persistent connections pinned to servers. Squid
would discover that the pinned server connection is closed only after
receiving a new request on the idle client connection and trying to write that
request to the server. In such cases, Squid propagates the pinned connection
closure to the client (as it should).

Chrome and, to a lesser extent, Firefox handle such races by opening a new
connection and resending the failed [idempotent] request transparently to the
user. However, IE usually displays an error page to the user.

While some pconn races cannot be avoided, without monitoring idle pconns,
Squid virtually guaranteed such a race in environments where origin server
idle connection timeout is smaller than client/Squid timeouts and users
are revisiting pages in the window between those two timeouts.

Squid now monitors idle pinned connections similar to idle connections in the
pconn pool and closes the corresponding idle client connection to keep the two
sides in sync (to the extent possible).

It is theoretically possible that this change will break servers that send
whitespace on an idle persistent connection or perhaps send some SSL keepalive
traffic. No such cases are known to exist though.