Do not invoke LinkPersister#isLinkContent for attributes

As in this case, we KNOW it's an attribute, and attribute
content is what we are interested in.
Walker and SnapRemover improvements.

Changes:
* walker made able to perform "breadth-first" walks
* snap remover made able to reuse items from walker
Fixing mocked UTs for NPE

As new call is added now for UID
Added test verifying that attribute files are not checked for being a link
Removed printlns and added Walker test for breadth-first
javadoc cleanup for RemoteRepositoryStorage: there is no uid param
Removal of Plx JT, using HTH in ITs
[NX-358] General System Settings
[NX-355] HTTP System Settings
[NX-355] Fix max port value
[NX-354] Notification settings
Fixing paths and decoupling
[NX-356] Security Realm Settings
[NX-354] Drop useless method
[NX-357] Anonymous security settings
getTestFile to not return null

If file does not exists. Use file.exists or file.isFile
to check it's existence (as already done everywhere except
on one place). On many other affected places, the code
is not even prepared for nulls, being NPE prone, so this
change is not affecting them.
Using new builder iface
Fixing props
Fix props for port
Fix properties