and external lock's on partitioned tables with many partitions.

- Added coverage tests.
- Added second prune_partitions call in mysql_update,
  to prune after subqueries and stored programs after
  locking.
- Fixed the problem with avoiding the second SEL_TREE creation,
  in the second prune_partitions() call, for conditions that did not
  have any subqueries or stored functions.
- Updated copyright year.
- some minor cleanup.