+act,sam,doc #3940 Added receive setter for Java Lambda actors

* Added a setter for Java lambda actors to "hide" the not so nice looking type signature of the "receive" method.
* Updated docs to reflect the changes.
* Converted samples to use the new setter.
Merge pull request #2087 from akka/wip-3940-add-sugar-to-abstract-actor-ban

+act,sam,doc #3940 Added receive setter for Java Lambda actors