Bug #1957 fix: Close a persistent ICAP connection if we have to open a

Bug #1957 fix: Close a persistent ICAP connection if we have to open a
new connection because the transaction is not retriable.

This change avoids a growing number of open connections when many
transactions create persistent connections but only few are retriable
and can reuse them.

FwdState was already doing that. I moved FwdState logic to
PconnPool::pop so that any PconnPool user thinks about the problem and
benefits from the common solution. The change should have no material
affect on FwdState.