API for Increasing Clone Data Size Estimates

The current version of this page is at https://github.com/facebook/mysql-5.6/wiki/API-for-Increasing-Clone-Data-Size-Estimates

The current clone implementation, once the first storage engine (which is always InnoDB) set the data size estimate for a clone stage, there is no way for other SEs to change that estimate to include their data estimates.

This is addressed by extending clone plugin callback with a new add_to_data_size_estimate(uint64_t estimate_delta) callback for non-InnoDB storage engines to use.

TODO: this it not final yet.