Some things still fail in innodb-index.test, and there seems to be
a race condition (data dictionary lock wait) when running with --valgrind.

dfield_t: Add an "external storage" flag, dfield->ext.

dfield_is_null(), dfield_is_ext(), dfield_set_ext(), dfield_set_null():
New functions.

dfield_copy(), dfield_copy_data(): Add const qualifiers, fix in/out comments.

data_write_sql_null(): Use memset().

big_rec_field_t: Replace byte* data with const void* data.

ut_ulint_sort(): Remove.

upd_field_t: Remove extern_storage.

upd_node_t: Replace ext_vec, n_ext_vec with n_ext.

row_merge_copy_blobs(): New function.

row_ins_index_entry(): Add the parameter "ibool foreign" for suppressing
foreign key checks during fast index creation or when inserting into
secondary indexes.

btr_page_insert_fits(): Add const qualifiers.

btr_cur_add_ext(), upd_ext_vec_contains(): Remove.

dfield_print_also_hex(), dfield_print(): Replace if...else if with switch.
Observe dfield_is_ext().