Fix orchestrator has already completed issue

[kaibocai]

Issue describing the changes in this PR

resolves 138

Pull request checklist

• [X] My changes do not require documentation changes
  • [ ] Otherwise: Documentation issue linked to PR
• [X] My changes are added to the CHANGELOG.md
• [X] I have added all required tests (Unit tests, E2E tests)

Additional information

Additional PR information

For a sample orchestration function

    @FunctionName("EternalOrchestrator")
    public void startWorkflowSchedule(@DurableOrchestrationTrigger(name = "runtimeState") TaskOrchestrationContext ctx) 
     {
        LOGGER.info("Doing something in the cleanup loop");

        ctx.createTimer(Duration.ofSeconds(60)).await();

        ctx.continueAsNew(null);
     }

The invocation process complete twice:

1. First time at https://github.com/microsoft/durabletask-java/blob/4a204cd238849d183581f7b73bcf86691ce97436/client/src/main/java/com/microsoft/durabletask/OrchestrationRunner.java#L66 ctx.complete(null)
2. Second time at https://github.com/microsoft/durabletask-java/blob/4a204cd238849d183581f7b73bcf86691ce97436/client/src/main/java/com/microsoft/durabletask/TaskOrchestrationExecutor.java#L60 This is the one causing exception as isCompleted flag is already set to true at the first complete.

Update the condition check at 2 to avoid repeating complete.