Allow unsafe transactions to save synchronously

CoreStore/Saving and Processing/UnsafeDataTransaction.swift

@@ -55,6 +55,17 @@ public final class UnsafeDataTransaction: BaseDataTransaction {
         }
     }
     
+    /**
+     Saves the transaction changes and waits for completion synchronously. For a `UnsafeDataTransaction`, multiple commits are allowed, although it is the developer's responsibility to ensure a reasonable leeway to prevent blocking the main thread.
+     
+     - returns: a `SaveResult` containing the success or failure information
+     */
+    public func commitAndWait() -> SaveResult {
+        
+        self.result = self.context.saveSynchronously()
+        return self.result
+    }
+    
     /**
      Rolls back the transaction.
      */