Combine save and create checkpoint into a single function in Contents

[KesterTong]

Right now, it's inconvenient to have save and create checkpoint be separate functions. This is because both functions are asynchronous, so our current implementation creates a race condition, and sometimes the previous save gets checkpointed.

This problem would be solved if we got rid of create_checkpoint and instead had a checkpoint option in save.

[KesterTong]

Actually I think that there is no race in the code, the problem was the Drive bug. Still, this might be worth considering anyway.