mysql_alter_table(): Consider ha_sequence::storage_ht() when determining
if the storage engine changed.
ha_sequence::check_if_supported_inplace_alter(): A new function, to
ensure that ha_innobase::check_if_supported_inplace_alter() will be
called on ALTER TABLE name_of_sequence SEQUENCE=0.
ha_innobase::check_if_supported_inplace_alter(): For any change of
the SEQUENCE attribute, always return HA_ALTER_INPLACE_NOT_SUPPORTED,
forcing ALGORITHM=COPY.
