[PATCH] Optimization: domain agnostic pconn

This removes the domain from the server pconn key.

Under the squid-3.2 pconn model the IP:port specifying the destination
are part of the key and can be used to strictly filter selection when
locating pconn. This means the domain is no longer a necessary part of
the key.

Squid using cache_peer can see a large number of wasted idle connections
to their peers due to the key domain value if the peer hostname is not
substituted properly. There is also a similar affect when contacting
servers with virtual hosted domains.

A simpler form of this with just the functional change in key generation
has been tested for several months now with only socket usage benefits
seen in a few production networks.

Amos