switch off dict_stats for the duration of bulk load

During a lengthy LOAD DATA INFILE operation found that dict_stats is taking more than half of CPU. This might be not critical, as the operation is IO bound, not CPU bound, but still, it makes sense to delay dict_stats on table till the end of bulk load.

Found when testing MDEV-33627 . Mostly likely, also older versions than 11.6 are affected, did not check.

See profiler output