br: allow log backup migration works with restore at the same time #62546
Description
Enhancement
If cluster A turned on log backup and doing a full restore, log backup will need to lock the log backup external storage and copy the SST from full restore. However, if cluster B is doing restore using cluster A's backup,. it also needs to get read lock for the same external storage, which leads to blocking. Two ops should be able to work at the same time and we could relax the locking