Avoid assertions on Range requests that trigger Squid-generated errors.

Avoid assertions on Range requests that trigger Squid-generated errors.

Added HttpRequest::ignoreRange() to encapsulate range ignoring logic.
Currently the new method only contains the code common among all callers. More
work is needed to check whether further caller homogenization is possible.

Documented that ClientSocketContext::getNextRangeOffset() may sometimes be
called before it is ready to do its job.