WAL retention do not ignore pinned backups

Given
```
B1----B2-----P----B3--->
set-backup -i P --ttl=30d
delete --delete-wal --wal-depth=2
```
Current result:
```
B1    B2      P----B3--->
```
Expected result:
```
B1    B2-----P----B3--->
```

Reported by Puchkov Vasily.