Wait checkpoint backup done in retention operation

In retention operation, the order is to create checkpoint and then delete old backups by rule, we should wait until the checkpoint create finished. Otherwise, it will cause the backup created newly to be excluded.

For example, we create a scheduledoperation which attribute 'max_backups' == 1:

1.The first time to execute operation, will create 1 checkpoint(A) and nothing to delete.

2.The second time to exeucte operation, will create 1 checkpoint(B), just at this time, if status of checkpoint(A) is 'available' and status of checkpoint is 'protecting', this results in not any backup being deleted, because only available checkpoint are considered.

This is unlogical for 'max_backups'.