NEXUS-5790: Content-Range support, resumable downloads

Changes:
* implemented limited content range support (only one range supported, but
any variety of it, like "X-", "X-Y" and even the non RFC but widespread
"-X").
* removed content-disposition, as it turned out this attribute was
not used. Only (restlet1x) resource using it was directly tampering
with Representation setting the needed flags on it.
* made servlet more resource conservative, getting file content
only when needed (in case of HEADs they were before read and
dumped by HttpServlet superclass) as getting input stream
is expensive op, and requires locking etc.