The method getDBForReadMirroring is passing a cmdObj that's provided from the `body` of the `request` that is parsed in the BasicCommandWithReplyBuilderInterface::Invocation.

That logic is odd and should be simplified.

We should see if we can simply return the _dbName directly since the cmdObj() parameter comes from the `request.body` which is used to create the DatabaseName internal member _dbName. 

Part of this change also include changing the return type from std::string to DatabaseName.