Remove old code that is not needed anymore with newer Netty versions.

This code also produced a deadlock with 3.6.0.Final and later as the VirtualExecutorService.awaitTermination(..) has a bug which is triggered by the new shutdown logic which was introduced with 3.6.0.Final.
Maven: update plugins
Remove spurious comments
Remove unused getter methods (which only make code inspection harder than necessary)
PagingManager and PagingStoreFactory do not use the PostOffice reference
Remove PostOffice.getPagingManager() as everywhere using it had access to server.getPagingManager()
Remove PagingStoreFactory.setStorageManager(...)