Update swf.executor #30: handle more then 1,000 open activity tasks

[ggreg]

Fix #30.

Ready for review! 📒

These changes introduce a private counter swf.executor._open_activity_count that counts the number of activity task that are scheduled or staged to be scheduled in the next decisions. The counter is incremented when a event is translated to a future or when there is no event and the executor needs to schedule one.

[ggreg]

pypy build is failing on:

=================================== FAILURES ===================================
______________________ test_workflow_with_child_workflow _______________________
    def test_workflow_with_child_workflow():
        workflow = TestDefinitionChildWorkflow
        executor = Executor(DOMAIN, workflow)

        history = builder.History(workflow,
                                  input={'args': (1,)})

        # The executor should schedule the execution of a child workflow.
        decisions, _ = executor.replay(history)
        assert len(decisions) == 1
        assert decisions == [{
            'startChildWorkflowExecutionDecisionAttributes': {
                'workflowId': 'workflow-test_workflow-1',
                'taskList': {
                    'name': 'test_task_list'
                },
                'executionStartToCloseTimeout': '3600',
                'input': '{"args": [1], "kwargs": {}}',
                'workflowType': {
                    'version': 'test_version',
                    'name': 'test_workflow'
                },
                'taskStartToCloseTimeout': '300'
            },
            'decisionType': 'StartChildWorkflowExecution'
        }]

        # Let's add the child workflow to the history to simulate its completion.
        (history
            .add_decision_task()
            .add_child_workflow(
                workflow,
                workflow_id='workflow-test_workflow-1',
                task_list=TestWorkflow.task_list,
                input='"{\\"args\\": [1], \\"kwargs\\": {}}"',
                result='4'))

        # Now the child workflow is finished and the executor should complete the
        # workflow.
        decisions, _ = executor.replay(history)
        workflow_completed = swf.models.decision.WorkflowExecutionDecision()
        workflow_completed.complete(result=json.dumps(4))

>       assert decisions[0] == workflow_completed
E       assert {'decisionTyp...st_version'}}} == {'decisionType...result': '4'}}
E         Detailed information truncated, use "-vv" to show
tests/test_dataflow.py:643: AssertionError
=============== 1 failed, 31 passed, 15 warnings in 3.87 seconds ===============
The command "py.test" exited with 1.
Done. Your build exited with 1.

[ggreg]

Merging and fixing pypy build in another issue.