Mark client streams that sent everything as STREAM_COMPLETE.

Mark client streams that sent everything as STREAM_COMPLETE.

The old code used STREAM_UNPLANNED_COMPLETE if the completed stream was
associated with a non-persistent connection, which did not make sense to me
and, IIRC, led to store entry aborts even though the entries were not damaged
in any way.

This change may expose other subtle bugs, but none are known at this time.

See also:
  http://www.squid-cache.org/mail-archive/squid-dev/200702/0017.html
  http://www.squid-cache.org/mail-archive/squid-dev/201102/0210.html