The creation of the primary key does not work.  We will have to flag
externally stored columns and copy the externally stored part from
the old table.

row_build_index_for_mysql(): Rename to row_merge_build_indexes().
Move from row0mysql.c to row0merge.c.

Remove private declarations from row0merge.h.  Make many functions static
in row0merge.c.

cmp_rec_rec_simple(): A new comparison function.

dict_index_get_min_size(): New function.

OS_FILE_FROM_FD(fd): A macro for converting from int to os_file_t.

rec_convert_dtuple_to_rec_comp(): Make the interface lower-level.

rec_get_converted_size_comp(): Return also extra_size.

UT_SORT_FUNCTION_BODY(): Remove reference to an obsolete test program.

row_rec_to_index_entry_low(): New function.

row0merge.c: Implement merge sort based on file streams instead of
fixed-size blocks.  Sort the small blocks as arrays of dfield_t*,
because it is faster than invoking rec_get_offsets() for every
comparison.