docs: add pgp key to repositories page
Plugins: Plugin failed to load since #8666

The method Path.endsWith(String s) doesn't work exactly the same way as String.endsWith() (see http://docs.oracle.com/javase/7/docs/api/java/nio/file/Path.html#endsWith(java.nio.file.Path)). This blocks the loading of plugins.
[ENGINE] Add engine lifecycle store reference to EngineHolder

This commit add the engines reference to the store out of the actual
implementation into the hodler since the holder manages the actual lifcycle.
Engine internal references like per searcher or per recovery are kept inside
the actual implemenation since the have a different lifecycle.