This option is known to be broken when tablespaces contain off-page
columns after crash recovery. It has only been tested when creating
the data files from the scratch.

btr_blob_dbg_t: A map from page_no:heap_no:field_no to first_blob_page_no.
This map is instantiated for every clustered index in index->blobs.
It is protected by index->blobs_mutex.

btr_blob_dbg_msg_issue(): Issue a diagnostic message.
Invoked when btr_blob_dbg_msg is set.

btr_blob_dbg_rbt_insert(): Insert a btr_blob_dbg_t into index->blobs.

btr_blob_dbg_rbt_delete(): Remove a btr_blob_dbg_t from index->blobs.

btr_blob_dbg_cmp(): Comparator for btr_blob_dbg_t.

btr_blob_dbg_add_blob(): Add a BLOB reference to the map.

btr_blob_dbg_add_rec(): Add all BLOB references from a record to the map.

btr_blob_dbg_print(): Display the map of BLOB references in an index.

btr_blob_dbg_remove_rec(): Remove all BLOB references of a record from
the map.

btr_blob_dbg_is_empty(): Check that no BLOB references exist to or
from a page. Disowned references from delete-marked records are
tolerated.

btr_blob_dbg_op(): Perform an operation on all BLOB references on a
B-tree page.

btr_blob_dbg_add(): Add all BLOB references from a B-tree page to the
map.

btr_blob_dbg_remove(): Remove all BLOB references from a B-tree page
from the map.

btr_blob_dbg_restore(): Restore the BLOB references after a failed
page reorganize.

btr_blob_dbg_set_deleted_flag(): Modify the 'deleted' flag in the BLOB
references of a record.

btr_blob_dbg_owner(): Own or disown a BLOB reference.

btr_page_create(), btr_page_free_low(): Assert that no BLOB references exist.

btr_create(): Create index->blobs for clustered indexes.

btr_page_reorganize_low(): Invoke btr_blob_dbg_remove() before copying
the records. Invoke btr_blob_dbg_restore() if the operation fails.

btr_page_empty(), btr_lift_page_up(), btr_compress(), btr_discard_page():
Invoke btr_blob_dbg_remove().

btr_cur_del_mark_set_clust_rec(): Invoke btr_blob_dbg_set_deleted_flag().

Other cases of modifying the delete mark are either in the secondary
index or during crash recovery, which we do not promise to support.

btr_cur_set_ownership_of_extern_field(): Invoke btr_blob_dbg_owner().

btr_store_big_rec_extern_fields(): Invoke btr_blob_dbg_add_blob().

btr_free_externally_stored_field(): Invoke btr_blob_dbg_assert_empty()
on the first BLOB page.

page_cur_insert_rec_low(), page_cur_insert_rec_zip(),
page_copy_rec_list_end_to_created_page(): Invoke btr_blob_dbg_add_rec().

page_cur_insert_rec_zip_reorg(), page_copy_rec_list_end(),
page_copy_rec_list_start(): After failure, invoke
btr_blob_dbg_remove() and btr_blob_dbg_add().

page_cur_delete_rec(): Invoke btr_blob_dbg_remove_rec().

page_delete_rec_list_end(): Invoke btr_blob_dbg_op(btr_blob_dbg_remove_rec).

page_zip_reorganize(): Invoke btr_blob_dbg_remove() before copying the records.

page_zip_copy_recs(): Invoke btr_blob_dbg_add().

row_upd_rec_in_place(): Invoke btr_blob_dbg_rbt_delete() and
btr_blob_dbg_rbt_insert().

innobase_start_or_create_for_mysql(): Warn when UNIV_BLOB_DEBUG is enabled.

rb://550 approved by Jimmy Yang