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.
Merge pull request #760 from normanmaurer/shutdown_hang

Remove old code that is not needed anymore with newer Netty versions.
Maven: update plugins
Remove spurious comments
rename parameter to avoid confusion
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(...)
Remove hard coded version. Use "Version".