Use case: to download reports for ALL accounts under the top manager account, we do:

1. Create an AdWordsSession object from top manager account
2. For each AdWords account, spawn a thread
3. In the thread clone the AdWordsSession object, call setClientCustomerId() with that AdWords account ID.
4. Use this AdWordsSession to invoke relevant API

We need a copy constructor in step 3. Our copy implementation cannot guarantee thread safety (in theory).