loop over data property to update only necessary persistedData keys

[christophe-g]

Similar to FirebaseExtended/polymerfire#166.

Also added a disconnected behavior to handle cases where data and persistedData should not be linked anymore, eg, for a case like data bound to a remote inaccessible resource:

<--! {{liveData}} === {} when [[user.uid]] is not set. -->
<firebase-document path="/userData/appCurrent/[[user.uid]]" data="{{liveData}}"></firebase-document>
<app-indexeddb-mirror log disconnected="[[!user.uid]]" session=[[user.uid]] key="appCurrent" data="{{liveData}}" persisted-data="{{data}}"></app-indexeddb-mirror>