Unfortunately, on Windows, Xamarin, and Android you can only commit one record at a time on the local MobileServicesSQLiteStore. This is because changes need to be made one at a time using methods like SyncTable.UpdateAsync, which is how you mark a change as being tracked to send to the server.
On native iOS, the Mobile Services offline sync feature uses Core Data, which does support a form of transaction (https://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/CoreData/Articles/cdMemory.html). If you use these APIs, you still need to make calls to the Mobile Services SDK methods to track each individual change in the internal tracking tables.
Why do you need transactions on the client side? There might be a different way to achieve your goal.