[MERGE] Connection pinning patch

Hi all,

This patch fixes the bug 1632
(http://www.squid-cache.org/bugs/show_bug.cgi?id=1632)
It is based on the original squid2.5 connection pinning patch developed
by Henrik (http://devel.squid-cache.org/projects.html#pinning) and the
related squid 2.6 connection pinning code.

Although I spend many hours looking on pined connections I am still not
absolutely sure that does not have bugs. However the code is very
similar with this in squid2.6 (where the pinning code runs for years)
and I hope will be easy to fix problems and bugs.

Regards,
     Christos