MongoDB to MongoDb

Our coding style defines names to be in CamelCase but this one was MIXEDCase.
The cache adapter wasn't released until now so this is no BC break but the session save handler and the log writer i thing. But normally PHP classes and methods are case-insensitive so this can only be an issue with some autoloaders on case-sensitive fs.

I think it's OK for the next minor release.

@Ocramius thoughts ?
typo : Dd to Db ...