[PATCH] Retry requests that failed due to a pconn race

Hello,

    The attached patch against trunk restores Squid ability to retry
connections that fail due to a pconn race condition. Currently, we serve
an ERR_ZERO_SIZE_OBJECT "Bad Gateway" error in those cases.

This patch is meant to reflect previous discussions on this topic but is
simpler/tighter than my previous attempts that were posted as PREVIEW
patches.

Thank you,

Alex.