Simplified Http::Message::parse() (#814)

Simplified Http::Message::parse() (#814)

size_t hdr_len cannot be negative, and sanityCheckStartLine() already
rejects zero values. We now use (and clearly document) the latter fact.

Also replaced a level-1 warning about "Too large" headers with a level-3
debugging message because huge headers is not a Squid problem, and the
problem should already be visible in access.logs records.