Clone the state map so that the StateMachineBuilder can be reused

[luozhenyu]

After the state machine is built, the state machine can still be modified through the previous builder. This PR fixes the problem.

StateMachineBuilder<States, Events, Context> builder = StateMachineBuilderFactory.create();
builder.externalTransition()
  .from(States.STATE1)
  .to(States.STATE2)
  .on(Events.EVENT1)
  .perform(doAction());
StateMachine<States, Events, Context> stateMachine = builder.build("StateMachineId");

builder.externalTransition()
  .from(States.STATE1)
  .to(States.STATE3)
  .on(Events.EVENT2)
  .perform(doAction());

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