Fix cache issue

[JiuqingSong]

To repro:

1. Open demo site, turn on side pane
2. Type "abc"
3. Type ENTER
4. Type BACKSPACE, keep doing it until everything is gone
5. Keep typing BACKSPACE

Expect: nothing happens
Actual, "ab" reappears

The root cause is demo site will always update content model by calling getContentModelCopy() which will update model cache before our formatContentModel() API write cache back, it is triggered when backspace. So the result is cached model in DOM tree is updated by demo site, but cached model in editor is updated by formatContentModel(), so the two models don't match. Later when we keep editing, it will edit on a staled cache.

To fix it, we should not update cache when create model from demo site. Furthermore, we should not update cache at all from API getContentModelCopy. It should be a "readonly" operation, to create model only, but not touch cache at all.

In this change I explicitly specified if a API call should reuse cache, and as long as any option is passed in, we should not update cache. For more detailed summary:

Scenarios	can use cache	can write cache	comment
getContentModelCopy: connected	true	false	Mostly used by demo site, we can use existing model but this should not impact cache
getContentModelCopy: disconnected	false	false	Used by plugins and test code to read current model. We will return a cloned model, and do not impact cache
getContentModelCopy: clean	false	false	Used by export HTML, do not use cache to make sure the model is up to date
formatInsertPointWithContentModel	false	false	Used by insertEntity (recent change), do not use cache since we need to add shadow insert point
getFormatState	true	false	We can reuse cache if we have, but when there is no cache, we will create reduced model so do not impact cache
other formatContentModel cases	true	true	Normal case, we can reuse cache, and should update cache

I added test cases to ensure the result of this table.

Also fix another recent regression in Editor.getContentModelCopy() with parameter "clean", we need to clone the content model to avoid removing entity DOM element when write back to DOM. Related test cases added.