into partitioned MyISAM table

Problem was that the ha_data structure was introduced in 5.1
and only used for partitioning first, but with the intention
of be of use for others engines as well, and when used by other
engines it would clash if it also was partitioned.

Solution is to move the partitioning specific data to a separate
structure, with its own mutex (which is used for auto_increment).

Also did rename PARTITION_INFO to PARTITION_STATS since there
already exist a class named partition_info, also cleaned up
some related variables.