Bug in context handling.

In short, virtual items had wrong context.
NEXUS-5790: Alternate implementation of content resource

To skip all the Restlet muck, using vanilla Servlet API.

Improvements:
* main focus is on download speed improvements
* got rid of Restlet, this is clean room reimplementation of content resource
* handling range requests (resumable downloads) (limited, only single range supported)
* 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.