Connection pinning (patch)

Hi,

Adrian asked me to check the connection pinning code in HEAD (as we're
actually using it on our network), and I can see a couple of problems.
I've attached a diff that should fix them.

The first part of this patch will make sure we only mark a requset with the
pinned and auth flags if there is a server-side persistent connection
waiting. This will stop extra server-side fd's from being marked as pinned.
The second part of the patch makes sure the correct code is followed after a
pconnPop() for pinned and tproxy connections.

Steven