Bug 1961 extra: Convert the URL::parse method API to take const URI strings

Bug 1961 extra: Convert the URL::parse method API to take const URI strings

The input buffer is no longer truncated when overly long. All callers have
been checked that they handle the bool false return value in ways that do
not rely on that truncation.

Callers that were making non-const copies of buffers specifically for the
parsing stage are altered not to do so. This allows a few data copies and
allocations to be removed entirely, or delayed to remove from error handling
paths.

While checking all the callers of Http::FromUrl several places were found to
be using the "raw" URL string before the parsing and validation was done. The
simplest in src/mime.cc is already applied to v5 r15234. A more complicated
redesign in src/store_digest.cc is included here for review. One other marked
with an "XXX: polluting ..." note.

Also, added several TODO's to mark code where class URL needs to be used when
the parser is a bit more efficient.

Also, removed a leftover definition of already removed urlParse() function.