Add an index to the `file_store` column on `lfs_objects`. This makes
counting local objects faster.
Also, there is no longer need to check for objects with `file_store`
being `NULL`. See
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18557
---
### Query plans
#### Before & with `NULL`
```
Aggregate (cost=113495.96..113495.97 rows=1 width=8) (actual time=1691.394..1691.394 rows=1 loops=1)
-> Seq Scan on lfs_objects (cost=0.00..106415.50 rows=2832186 width=0) (actual time=0.012..1312.488 rows=2852607 loops=1)
Filter: ((file_store = 1) OR (file_store IS NULL))
Rows Removed by Filter: 131
Planning time: 0.077 ms
Execution time: 1691.433 ms
```
#### Before, without `NULL`
```
Aggregate (cost=113495.96..113495.97 rows=1 width=8) (actual time=856.423..856.424 rows=1 loops=1)
-> Seq Scan on lfs_objects (cost=0.00..106415.50 rows=2832186 width=0) (actual time=0.012..672.181 rows=2852607 loops=1)
Filter: (file_store = 1)
Rows Removed by Filter: 131
Planning time: 0.128 ms
Execution time: 856.470 ms
```
#### After & with `NULL`
```
Aggregate (cost=68819.95..68819.96 rows=1 width=8) (actual time=583.355..583.355 rows=1 loops=1)
-> Index Only Scan using index_lfs_objects_on_file_store on lfs_objects (cost=0.43..61688.35 rows=2852643 width=0) (actual time=0.028..399.177 rows=2852607 loops=1)
Filter: ((file_store = 1) OR (file_store IS NULL))
Rows Removed by Filter: 131
Heap Fetches: 867
Planning time: 0.096 ms
Execution time: 583.404 ms
```
#### After, without `NULL`
```
Aggregate (cost=68817.29..68817.30 rows=1 width=8) (actual time=490.550..490.551 rows=1 loops=1)
-> Index Only Scan using index_lfs_objects_on_file_store on lfs_objects (cost=0.43..61685.68 rows=2852643 width=0) (actual time=0.040..311.760 rows=2852607 loops=1)
Index Cond: (file_store = 1)
Heap Fetches: 831
Planning time: 0.294 ms
Execution time: 490.590 ms
```
Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/6067
Old records have a null value of file_store column.
This causes the problems with current direct_upload implementation,
as this makes it to choose Store::REMOTE instead of Store::LOCAL.
This change moves the store save when change saving the object.
