ConcurrentModificationException iterating Backstack

Hi! I'm seeing the following. The flow is from a conductor activity, to a dialog, the calling out to the SMS app, and then when the user hits the back button from the native SMS app, we get the following:

```
java.util.ConcurrentModificationException
at java.util.ArrayDeque$DeqIterator.next(ArrayDeque.java:608)
at com.bluelinelabs.conductor.Router.onActivityStarted(Router.java:532)
at com.bluelinelabs.conductor.internal.LifecycleHandler.onActivityStarted(LifecycleHandler.java:334)
at android.app.Application.dispatchActivityStarted(Application.java:205)
at android.app.Activity.onStart(Activity.java:1150)
at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:564)
at android.support.v7.app.AppCompatActivity.onStart(AppCompatActivity.java:177)
at com.locationlabs.cni.mvp.BaseControlsActivity.onStart(BaseControlsActivity.java:51)
at com.locationlabs.cni.contentfiltering.AppControlsActivity.onStart(AppControlsActivity.java:117)
at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1237)
at android.app.Activity.performStart(Activity.java:6268)
at android.app.Activity.performRestart(Activity.java:6314)
at android.app.Activity.performResume(Activity.java:6319)
at android.app.ActivityThread.performResumeActivity(ActivityThread.java:3092)
at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:3134)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1388)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
```

can you make the Router's Backstack iterator thread-safe, or, provide any other advice? as a workaround, I'm simply catching it and moving on...

```
@Override
protected void onStart() {
try {
super.onStart();
getPresenter().onViewStarted();
} catch (RuntimeException e) {
Log.dw("we still need a proper fix for ");
}
}
```

tried
conductor_version=2.1.4
and
conductor_version=2.1.5-SNAPSHOT

Start in Router.java at onActivityStarted around line 532 and trace the Backstack iteration alongside lifecycle callbacks in LifecycleHandler.java around line 334. Reproduce the activity return from the native SMS app if possible, then verify that backstack changes during this lifecycle path no longer produce ConcurrentModificationException.

Written by the indexing model from the issue text.