Introduce the configuration option innodb_log_optimize_ddl
for controlling whether native index creation or table-rebuild
in InnoDB should keep optimizing the redo log
(and writing MLOG_INDEX_LOAD records to ensure that
concurrent backup would fail).

By default, we have innodb_log_optimize_ddl=ON, that is,
the default behaviour that was introduced in MariaDB 10.2.2
(with the merge of InnoDB from MySQL 5.7) will be unchanged.

BtrBulk::m_trx: Replaces m_trx_id. We must be able to check for
KILL QUERY even if !m_flush_observer (innodb_log_optimize_ddl=OFF).

page_cur_insert_rec_write_log(): Declare globally, so that this
can be called from PageBulk::insert().

row_merge_insert_index_tuples(): Remove the unused parameter trx_id.

row_merge_build_indexes(): Enable or disable redo logging based on
the innodb_log_optimize_ddl parameter.

PageBulk::init(), PageBulk::insert(), PageBulk::finish(): Write
redo log records if needed. For ROW_FORMAT=COMPRESSED, redo log
will be written in PageBulk::compress() unless we called
m_mtr.set_log_mode(MTR_LOG_NO_REDO).