Encapsulate calculations of the table access cost and make them consistent and tunable

This task is mostly a re-engineering task.
Currently when calculating the cost of a ref table access method the function best_access_path makes an assumption that any index look-up is performed into a B-tree and the function uses engine independent code for this calculations.
The cost calculations also use some constants that should be done tunable through system variables tables since these constants may be different for different platform/environments.

This task is implemented in preparation for the development that will support more realistic cost model for query execution plans.

[1] http://askmonty.org/worklog/?tid=251

This is a prerequisite for MDEV-350 and similar to what MySQL 5.7 has under the name "Optimizer Cost Model"