[PATCH] ICAP pconn optimization

 ICAP services use a "service" model of pconn different from the "TCP
 destination" model which PconnPool objects are designed for.

 This patch alters Adaptation::Icap::ServiceRep to use the simpler
 IdleConnList object for pconn storage. IdleConnList stores a "set of
 idle connections" more compatible with the ICAP model.

 In order to implement ICAP max-connections feature the closeN()
 operation is added to IdleConnList.

 The result is removal of the complex hash and management operations on
 push/pop of the idle conn set. The only expected behaviour change is
 more frequent re-use of idle connections on services with multiple IP
 addresses. Speed gains are minimal, but positive.

 Amos