Bug 2034 fix, v4: Separate handling of virgin server responses and ICAP-adapted

Bug 2034 fix, v4: Separate handling of virgin server responses and ICAP-adapted
responses. Both can be active at the same time. Only one can become final and
get into the store entry.

Also, do not abort ICAP transactions when the virgin response ends prematurely.
Keep going because the ICAP initiator may still want to know the ICAP reaction
to the now-truncated virgin message.

This version passes a few basic tests, but may need more work and does need
polishing. For example, the common code should be factored out from
ICAPModXact::noteBodyProductionEnded and ICAPModXact::noteBodyProducerAborted.