master: fix possibly wrong knowledge of cells' backup_tid when resuming backup
The issue happens when there were commits while the backup cluster was down. In this case, the master thinks that these commits are already replicated, reporting wrong backup_tid to neoctl. It solved by itself once: - there are new commits triggering replication for all partitions; - all storage nodes have really replicated. This also resulted in an inconsistent database when leaving backup mode during this period.
Showing with 38 additions and 7 deletions