NEXUS-5472: Simplification

First pass.
* Local content discover is done by Walker directly, no LocalStrategies anymore
* removed unneeded LocalStrategy imples, as LocalContent discoverer now directly implements Walks
* Strategy made RemoteStrategy, used by Remote content discovery only
* merge entry source simplified, only retains uniqueness
NEXUS-5472: Javadoc fix
NEXUS-5472: Javadoc
NEXUS-5472: Fix rebuild case

WL should be replaced IF exists and is built.
Before building, the WL should not be dropped, as
it might cause a request to "slip in" that would be
blocked by old and also by new WL (while rebuilding it)