Only add indexed BLOBs to row_ext.
trx_undo_rec_get_partial_row(): Move the BLOB fetching to row_ext_create().
row_build(): Pass only those BLOBs to row_ext_create() that are referenced by
ordering columns of some indexes, similar to trx_undo_rec_get_partial_row().
row_ext_create(): Add the parameter "tuple". Move the implementation
from row0ext.ic to row0ext.c.
row_ext_lookup_ith(), row_ext_lookup(): Return a const pointer. Remove
the parameters "field" and "f_len". Make the row_ext_t* parameter const.
row_ext_t: Remove the field zip_size.
field_ref_zero[]: Declare in btr0types.h instead of btr0cur.h.
row_ext_lookup_low(): Rename to row_ext_cache_fill() and change the
signature.
inserted, uncommitted clustered index records when determining if a
secondary index record that contains a column prefix of an externally
stored column is referencing the clustered index record.
field_ref_zero[]: A BLOB pointer full of zero, for use in comparisons.
btr_copy_externally_stored_field_prefix(): Assert that the BLOB pointer is set.
row_ext_lookup_ith(), row_ext_lookup(), row_ext_lookup_low(): Document
that field_ref_zero is returned when the BLOB cannot be fetched.
row_ext_lookup_low(): Return field_ref_zero and *len = 0 when the
BLOB pointer is unset.
row_build_index_entry(): Return NULL when a needed BLOB pointer cannot
be dereferenced (row_ext_lookup returns field_ref_zero). Check the
return value for NULL in callers.
row_vers_impl_x_locked_off_kernel(): Avoid comparisons when
row_build_index_entry() returns NULL.
row_vers_old_has_index_entry(): Ignore records for which
row_build_index_entry() returns NULL. The entry should never be NULL
in rollback, but it may be NULL in purge.
row_merge_buf_add(): Assert that row_ext_lookup() does not return
field_ref_zero. The table will be locked during index creation.
This will be needed for fixing Bug #22496.
REC_MAX_INDEX_COL_LEN: New constant, copied from DICT_MAX_INDEX_COL_LEN.
row_ext_create(), row_ext_lookup(), row_ext_lookup_low(): New functions.
