Invalidate cursors if the connection was dropped

When a connection was closed or died, cursor should be invalidated/closed too.

>>> conn=mariadb.connection(user="root")

>>> cursor=conn.cursor()

>>> print(cursor.closed)

False

>>> conn.close()

>>> print(cursor.closed)

False

That means closed should be implemented as getter/setter function and should check the following conditions:

• is connection alive
• is statement valid
• is statement error code==CR_STMT_CLOSED

Additionally the MARIADB_CHECK_STMT macro should also check if the error code is CR_STMT_CLOSED.