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.

Aligned bufSizes of container and Nexus.

Preferred way to set response buffer size is to tweak
the container (in case of bundle, Jetty). By default
Jetty uses 32KB buffer, and with this change Nx
will use it too. If this size is inappropriate,
you would want to tweak Jetty embedded in the bundle.

Still, if you need, you can tweak Nx response buffer
size, and Nx will try to pass this information to
the container too.

In bundle case, preferred way is to tweak Jetty,
while the latter mode gives you some freedom
if Nx deployed as WAR in a Servlet container.