This PR adds the ability to create a timer and allow a workflow to pause execution for a fixed amount of time. This uses delayed dispatching under the hood. Therefore, there is nothing actually running while a timer is active.

For example the following workflow will pause for 5 minutes and then complete:

class TestWorkflow extends Workflow
{
    public function execute()
    {
        yield WorkflowStub::timer(300);

        return;
    }
}

I will eventually make these timers cancelable.