Implement class Item_func_timestamp

Currently functions TIMESTAMP() and ADDTIME()/SUBTIME() reuse the same class Item_func_add_time.
We'll add a dedicated class for TIMESTAMP().

Rationale:

• Item_func_timestamp does not have to derive from Item_hybrid_func. It always returns a fixed data type (DATETIME for now).
• Soon we'll change TIMESTAMP() to return TIMESTAMP instead of DATETIME, so the underlying implementations will diverge anyway.
• Item_func_add_time will be simplified

Note, with the recently implemented class Sec6_add, there won't be duplicate code.