simplify concurrency model

elevator-core/src/main/java/elevator/Building.java

elevator-core/src/main/java/elevator/Clock.java

elevator-core/src/main/java/elevator/ClockListener.java

...r-core/src/main/java/elevator/ui/BuildingAndElevator.java

elevator-core/src/main/java/elevator/ui/ElevatorUI.java

elevator-server/pom.xml

...er/src/main/java/elevator/server/ElevatorApplication.java

...or-server/src/main/java/elevator/server/ElevatorGame.java

...-server/src/main/java/elevator/server/ElevatorServer.java

...or-server/src/main/java/elevator/server/HTTPElevator.java

...r-server/src/main/java/elevator/server/ServerStarter.java

...erver/src/test/java/elevator/server/ElevatorGameTest.java

...ver/src/test/java/elevator/server/ElevatorServerTest.java

...erver/src/test/java/elevator/server/HTTPElevatorTest.java