POC: LevelDB AttributeStorage (experimental)

A fully workable Nexus OSS with AttributeStorage that
uses LevelDB

Experimental! No upgrade path for example.... works for
new deployments, but upgraded instance will not have
attributes upgraded!

* introduced LevelDB "provider" plugin with some helpers
* LevelDB AttributeStorage moved out (just as LevelDB itself) from core
* LevelDB AttributeStorage is now configurable via nexus.properties (default is LS)