Sync Gateway: Expose Mobile Metadata Sync TimeStamp

There are cases where it would be relevant for apps to identify the time when the document was synced by Sync Gateway. This is the timestamp corresponding to when the mobile metadata was updated by Sync Gateway (timestamps for 'import time' or 'SG write time' )

Use cases for this could be

• traceability of document as it flows through the user's backend pipeline starting with the time the document was synced to the backend server.
• TS based Conflict resolution based on when document was written to server

It is not possible for apps to include a user defined TS field as the TS would correspond to the time when document was created. With client partitioning, network delays and no guarantee of time sync across clients, this TS would not correspond to when the document was actually synced by the Sync Gateway