when creating table

Federated tables had an artificially low maximum of key length, 
because the handler failed to implement a method to return it and 
the default value is taked from the prototype handler.

Now, implement that method and return the maximum possible key
length, which is that of InnoDB.


mysql-test/r/federated.result:
  Verify that unique keys may be longer than 255 characters.
mysql-test/t/federated.test:
  Verify that unique keys may be longer than 255 characters.
sql/ha_federated.h:
  Implement the virtual method that tells the max size of parts to 
  make a key.
  
  Backport the length defined in 5.1.