[PATCH] Pipeline prefetch build-time control

The attached patch adds a built-time setting SQUID_PIPELINE_MAX to
replace the magic numbers previously used for limiting the amount of
prefetch Squid does when pipeline_prefeth config directive is enabled.
The default remains at 2 unless altered by using -DSQUID_PIPELINE_MAX=n
in CXXFLAGS and this adds some documentation about why, and what can be
done to improve pipelining in future.

Amos