with-items concurrency not respected for join task

[Winson Chan]

For the following workflow, concurrency is not respected for task3 that joins on task1 and task2. If there is no join, concurrency works fine.

version: '2.0'

wf1:
    type: direct

    tasks:
        task1:
            action: std.noop
            on-success:
                - task3
        task2:
            action: std.noop
            on-success
                - task3
        task3:
            join: all
            with-items: i in <% list(range(0, 6)) %>
            concurrency: 2
            action: std.sleep
            input:
                seconds: 3