when validating vcol's (default, check, etc) in ALTER TABLE
vcol_info->flags are modified in place. This means that if ALTER TABLE
fails for any reason we need to restore them to their original values.
(mroonga was freeing the memory on ::reset() but not on ::close())
* invoke check_expression() for all vcol_info's in
mysql_prepare_create_table() to check for FK CASCADE
* also check for SET NULL and SET DEFAULT
* to check against existing FKs when a vcol is added in ALTER TABLE,
old FKs must be added to the new_key_list just like other indexes are
* check columns recursively, if vcol1 references vcol2,
flags of vcol2 must be taken into account
* remove check_table_name_processor(), put that logic under
check_vcol_func_processor() to avoid walking the tree twice
InnoDB was too eager to forget the open table (m_mysql_table=NULL)
and that caused it to try to open a table which was opened by the user
not FK-prelocked. The server didn't expect that.
After fixing this, it crashed in gcol.innodb_virtual_fk test, trying to
compute virtual columns for a table that didn't have them. Because
row_upd_store_row() was deleting a row from node->table, while computing
virtual columns in thr->prebuilt->m_mysql_table. Which wasn't necessarily
the same table, and might've not even had virtual columns, even if
node->table did.
