Derivative fields have to listen for sub collections changes

[n-sviridenko]

We have a sub collection we want to listen to in order to update its parent document field value. This is needed in order to not compute its value manually every time visiting that sub collection, which takes a lot of time.

Sample use-case: summing up amounts of cart items and writing it down to the cart, which is a parent document to them.

Unfortunately, currently derivative fields only consider the current document changes.