Upgrade ICAP persistent connection handling

Upgrade ICAP persistent connection handling

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.