Cleanup: replace urlCanonical() with HttpRequest::effectiveReuqestUri()

Cleanup: replace urlCanonical() with HttpRequest::effectiveReuqestUri()

We have previously been using the term "canonical URL" in Squid to mean
absolute-URI, but not in all cases and may sometimes mean authority-form.
RFC 7230 introduces a new term "Effective Request URI" which directly
matches our desired usage.

* make urlCanonical() global function a method of class HttpRequest
  since it depends on request method for its particular form syntax

* remove the now unnecessary canonical member and HttpRequest::SetHost()

* convert HttpRequest::storeId(), Ftp::UrlWith2f(), and ps_state::url()
  to SBuf usage to avoid performance regressions in their use.

* replace many uses of xstrdup() with xstrndup() for performance where
  the copy cannot be avoided entirely.

* avoid using urlParse() to do a simple URL data-copy in ICAP handling

* update stub_HttpRequest.cc to match full class HttpRequest API