added THD::work_part_info member where we now store modified
partition_info structure.
It allows no solve problem when different parts of the part_info get
into different mem_roots


sql/partition_info.cc:
  get_clone implementation
sql/partition_info.h:
  get_clone() declared
sql/sql_class.h:
  THD::work_part_info added
sql/sql_partition.cc:
  thd->work_part_info instead of lex->part_info
sql/sql_table.cc:
  thd->work_part_info instad of lex->part_info
sql/unireg.cc:
  thd->work_part_info instad of lex->part_info