- current_arena to stmt_arena: the thread may have more than one
'current' arenas: one for runtime data, and one for the parsed 
tree of a statement. Only one of them is active at any moment.
- set_item_arena -> set_query_arena, because Item_arena was renamed to 
Query_arena a while ago
- set_n_backup_item_arena -> set_n_backup_active_arena;
the active arena is the arena thd->mem_root and thd->free_list
are currently pointing at.
- restore_backup_item_arena -> restore_active_arena (with the same
rationale)
- change_arena_if_needed -> activate_stmt_arena_if_needed; this
method sets thd->stmt_arena active if it's not done yet.


sql/item.cc:
  Rename.
sql/item_cmpfunc.cc:
  Rename.
sql/item_func.cc:
  Rename.
sql/item_subselect.cc:
  Rename.
sql/item_subselect.h:
  Remove an unused forward declaration.
sql/item_sum.h:
  Remove an unused forward declaration.
sql/mysql_priv.h:
  Remove an unused forward declaration.
sql/sp.cc:
  Rename.
sql/sp_head.cc:
  Rename.
sql/sql_base.cc:
  Rename.
sql/sql_class.cc:
  Rename.
sql/sql_class.h:
  Rename.
sql/sql_lex.cc:
  Rename.
sql/sql_parse.cc:
  Rename.
sql/sql_prepare.cc:
  Rename.
sql/sql_select.cc:
  Rename.
sql/sql_show.cc:
  Rename.
sql/sql_union.cc:
  Rename.
sql/sql_view.cc:
  Rename.
sql/table.cc:
  Rename.