Fix copying blobs that are not included in snapshot. (#558)

The issue was that we weren't setting the snapshot for the query that
fetches the list of large object ids. This inconsistency led to errors when
trying to access large objects that weren't restored on target.

Error:
```
15:30:11 108 ERROR  [SOURCE 85] ERROR:  large object 16390 does not exist
15:30:11 108 ERROR  [SOURCE 85] Context: Failed to open large object 16390
15:30:11 107 ERROR  [SOURCE 83] ERROR:  large object 16392 does not exist
15:30:11 107 ERROR  [SOURCE 83] Context: Failed to open large object 16392
```