of externally stored columns to 108 bytes when possible.

This will unmask a bug in the handling of BLOBs on compressed tablespaces,
especially of small page sizes.  Apparently, the locally stored part must
be less than 235 bytes in size for the bug to occur.

dict_col_struct: Add field min_prefix.

dict_index_build_internal_non_clust(): Adjust min_prefix.