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.
NEXUS-5790: Transfer improvement.

ContentLocator iface changed, added new method that is
in case of FileContnetLocator implemented using zero-copy.

This solution performs as fast as Jetty's native handler.