+per #3906 add Java8 support for Persistence

- Provided new interfaces for akka-persistence to be usable directly
  through ReceiveBuilder/PartialFunction. Added a sample java project to
  showcase the usage of these API's with akka-persistence.
- Fixed a minor comment block in javadoc code snippet.
- Renamed java event persistor and fixed a documentation typo.
- Put back java event persistence methods in
  UntypedEventsourcedProcessor and copied them into
  AbstractEventsourcedProcessor for the sake of clarity in javadocs.
  Also corrected some doc punctuations.
- Documentation for akka-persistence java 8 lambda expressions support.
- Moved code examples referred from within lambda-persistence.rst to
  java8 compatible sample project.
- Removed remaining unwanted java8 compatible source files.
+per #3906 use AbstractActor for Java8 Persistence

- also rename and link doc chapter
Merge pull request #2056 from akka/wip-persistent-java8-∂π

Wip persistent java8 ∂π