row_search_for_mysql(): When a secondary index record might not be
visible in the current transaction's read view and we consult the
clustered index and optionally some undo log records, return the
relevant columns of the clustered index record to MySQL instead of the
secondary index record.

REC_INFO_DELETED_FLAG: Move the definition from rem0rec.ic to rem0rec.h.

ibuf_insert_to_index_page_low(): New function, refactored from
ibuf_insert_to_index_page().

ibuf_insert_to_index_page(): When we are inserting a record in place
of a delete-marked record and some fields of the record differ, update
that record just like row_ins_sec_index_entry_by_modify() would do.

mysql_row_templ_t: Add clust_rec_field_no.

row_sel_store_mysql_rec(), row_sel_push_cache_row_for_mysql(): Add the
flag rec_clust, for returning data at clust_rec_field_no instead of
rec_field_no. Resurrect the debug assertion that the record not be
marked for deletion. (Bug #55626)

buf_LRU_free_block(): Refactored from
buf_LRU_search_and_free_block(). This is needed for the
innodb_change_buffering_debug diagnostics.

[UNIV_DEBUG || UNIV_IBUF_DEBUG] ibuf_debug, buf_page_get_gen(),
buf_flush_page_try():
Implement innodb_change_buffering_debug=1 for evicting pages from the
buffer pool, so that change buffering will be attempted more
frequently.