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.