feat(document): add $createModifiedPathsSnapshot(), $restoreModifiedPathsSnapshot(), $clearModifiedPaths()

[vkarpov15]

Fix #14268

Summary

$createModifiedPathsSnapshot() takes a snapshot of the document's current change tracking state, without modifying the document's properties. $restoreModifiedPathsSnapshot() restores the document's change tracking state to a given snapshot.

Primarily useful for custom transaction wrappers like #14268. When you modify a document in a transaction and that document was created outside of the transaction, you may need to restore the document's change tracking state if the transaction was aborted.

As part of making this work, I also moved Model.prototype.$__delta and supporting code to Document.prototype.$__delta. Otherwise, getChanges() crashes on subdocuments because subdocuments don't have a $__delta function.

Examples

[hasezoey]

LGTM, some minor suggestions

[hasezoey]

Suggested change

	* The function profile can look like:
	* this.forEach(state1, fn); // iterates over all paths in state1
	* this.forEach(state1, state2, fn); // iterates over all paths in state1 or state2
	* this.forEach(fn); // iterates over all paths in all states
	*
	* @param {String} [state]
	* @param {String} [state]
	* @param {Function} callback

from what i can still tell, this function does not take any arugments, does it? (also the documented code above has no relation to this function