MDEV-18577: Indexes problem on import dump SQL

Problem was that we skipped background persistent statistics calculation on applier nodes if thread is marked as high priority (a.k.a BF). However, on applier nodes all DDL which is replicate will be executed as high priority i.e BF.

Fixed by allowing background persistent statistics calculation on applier nodes even when thread is marked as BF. This could lead BF lock waits but for queries on that node needs that statistics.

Thanks to janlindstrom for https://github.com/mariadb/server/commit/e73060b6c9b3655cadcd34c66fc7811d214c1673