on partitioned tables with many partitions.


Work In Progress!

Splitted open_and_lock_tables() into open_query_tables() and
lock_query_tables().

Refactored mysql_select so that open_query_tables could be done
before the prepare step, so that prune_partitions could be done
before lock_query_tables() was called. lock_query_tables() is now
called before the optimize step.

Not yet done:
Implement pruning for INSERT.
Fix the last failing tests.