innodb-index.test: Add a test with a large number of externally stored
columns.  Check that there may not be prefix indexes on too many columns.

dict_index_too_big_for_undo(): New function: Check if the undo log may
overflow.

dict_index_add_to_cache(): Return DB_SUCCESS or DB_TOO_BIG_RECORD.
Postpone the creation and linking of some data structures, so that
when dict_index_too_big_for_undo() holds, it will be easier to clean up.
Check the return status in all callers.