OAK-662: Reduce boilerplate code in JCR impl methods

Add ItemImpl.perform() to allow adding item-specific functionality to it.
No need to explicitly pass SessionDelegate to constructors that already take an ItemDelegate.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1451334 13f79535-47bb-0310-9956-ffa450edef68