NEXUS-5790: New content resource implementation, part II

Motto: Cleanup of Items and ContentLocators

Continues on pull trail
https://github.com/sonatype/nexus-oss/pull/108

To compare diffs against pull above:
https://github.com/sonatype/nexus-oss/compare/nexus-5790-new-content-impl...nexus-5790-new-content-impl-2

Main intention of this pull is to 
* clean up items API
* clean up content related APIs, simplify things, content length not disjointed anymore with actual content
* properly handle "generated" and "virtual" items
* remove deprecated stuff, change existing code using removed methods
& added many Javadocs (on above affected areas)

CI
https://bamboo.zion.sonatype.com/browse/NX-OSSF3