Optimize btr_page_reorganize_low() redo logging

MDEV-12353 introduces a purely physical redo log format.

For btr_page_reorganize_low(), there is no optimization as of now. It is essentially copying the page payload to the redo log, as if all records were inserted anew. A better approach would be to compare the old page contents to the reorganized one, and only emit the minimal amount of WRITE or MEMMOVE records to cover the changes.

This affects especially the innodb_defragment feature, because it is invoking btr_page_reorganize_low() extensively. The test used to work with the mysql-test/include/default_mysqld.cnf se{{page_delete_rec_list_end()}}tting of innodb_log_buffer_size=1M, but that had to be increased to innodb_log_buffer_size=6M to avoid warnings about the buffer being extended.