NEXUS-5790: New content resource implementation

Cleanroom reimplementation of the `/content` resource using vanilla Servlet API, goal was to get rid of heaps of abstraction layers (Restlet). To skip all the Restlet muck, using vanilla Servlet API.

The 3 "views" ("describe", "info" and "maven2") implemented using redirects, as they used DTOs from restlet1x. This is tested and works just nice from browsers and in UI, only corresponding IT client needed a change to follow redirects.

ContentLocator interface remains unchanged. Testing shows that no (visible) gain from NIO happens (like using Channels or direct buffers), as on servlet side we already get "wrapped" ServletOutputStream from Jetty with heap buffers. On the other hand, Jetty pushes rather fast the content with increased buffer size (was 4kB now default is 16kB and is configurable).

Measured transfer speeds are in range of Jetty ResourceHandler range (excluding second pass, when Jetty's ResourceCache kicks in). For numbers, see issue (below).

Improvements and changes in short:
* main focus is on download speed improvements
* got rid of Restlet, this is clean room reimplementation of content resource using vanilla Servlet API
* 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.

Side-effect: fixed a bug in router regarding context handling.

Issue
https://issues.sonatype.org/browse/NEXUS-5790

CI:
https://bamboo.zion.sonatype.com/browse/NX-OSSF37-11