SyncService = function($http, LovefieldService) {
  this.http_ = $http;
  this.dataFetcher_ = new NetworkDataFetcher(this.http_);
  this.lovefieldService_ = LovefieldService;

  /** @private {?IThenable}*/
  this.whenInitialized_ = null;
}
A Serivce used for syncing the local DB with server.
@constructor

@param {!angular.$http} $http
@param {!LovefieldService} LovefieldService