OperationWrapper.java, AbstractOperation.java, Operation.java, PluginOperation.java:
In getAttachment(), removeAttachment() and setAttachment(), changed the return type to a generic type. This simplifies client code by removing the need to cast the returned value.

git-svn-id: https://svn.forgerock.org/opendj/trunk/opends@10996 41b1ffd8-f28e-4786-ab96-9950f0a78031