#554: Subclass Sandbox pattern (#998)

* Fix issue #761: ThreadSafeDoubleCheckLocking.java: Instantiating by Reflection call will be successful if you do that firstly

* Create leader election module

* Create Interface of Instance and MessageManager

* Create implementations with token ring algorithm

* Change package structure.
Create basic message system.

* Implement heartbeat and heartbeat invoking message system

* Implement election message handler

* Add leader message handler

* Add main entry point

* Add comments

* Update README.md

* Fix checkstyle issue

* Add Unit Tests

* Add Unit Tests

* Add bully leader selection

* Change System.out to log print.
Add MIT license in each file.

* Add More java doc comments

* Add unit test

* Add unit tests

* Add subclass-sandbox

* Add Unit Test

* Add Unit Test

* Fix Typo

* Move dependency into parent pom.xml

* Change local valuable reference to be var