stored column.  This is the first part of fixing Bug #22496.

btr_copy_externally_stored_field_prefix(): New function.

btr_copy_externally_stored_field(): Split to
btr_copy_externally_stored_field_prefix_low().

row_sel_sec_rec_is_for_blob(): New function, used by
row_sel_sec_rec_is_for_clust_rec() in selects via
a secondary index.