CXX-183 Add save method to dbclientinterface

We don't return an _id here (unlike other drivers) to be consistent with the rest of our decisions regarding _id and mutating documents for this version of the driver.

We could generate one and inject it here if one is not provided and return it if we want, just let me know.
Yeah, this file is 50/50 on whether they have em or not. We should come up with a rule. Spaces sounds good to me.