Sync: Add "Who started sync?" to sync status

In Automattic/wp-calypso#6695, @rickybanister suggested that it would be helpful to know who started the sync. As of right now, we're not storing that information on the Jetpack side. So, we would need to store the information in the status and then add it to the `/sites/$site/sync/status` endpoint.

From there, we could update the redux stores on the Calypso side.

I'm going to go ahead and milestone this for 4.4, since I know there are some other rough edges to smooth out on the Calypso side. But, cc @lezama to triage that and decide where to milestone.