and so the engine calls current_thd to derive transaction information;
instead we now pass THD to those functions, it looks more logical
(it makes the implicit current_thd parameter more visible).
Approved by Brian and Monty.


sql/handler.h:
  cursor's creation functions in the handlerton need a THD,
  it's better than have the engine call current_thd
sql/sql_cursor.cc:
  pass the THD instead of letting the engine call current_thd
storage/innobase/handler/ha_innodb.cc:
  use the passed THD instead of current_thd
storage/innobase/handler/ha_innodb.h:
  use the passed THD instead of current_thd