Firestore: collections updates

[merlinnot]

When a document in a collection is updated, a property is replaced with an entirely new array (see 2f5d602#diff-cd706bcc157b02ba8cbc5d4fb834a3eeR11). It forces elements such as iron-list, dom-repeat etc. to recreate all of its elements. I'd expect Firestore(Element|Mixin) to update only those documents which actually changed.