This worklog implements the initial version of a new Cost model API
for the optimizer. The existing hard coded cost constants are 
removed from the optimizer and handler code and moved into
the new cost model module.

Two new classes are implemented:
      
* Cost_model_server: This provides cost estimates for operations the
  server does that are not dependent the storage engine or storage
  device type.
      
* Cost_model_table: This provides cost estimates for basic operations
  on tables that can depend on the storage engine.