Enhance performance of counting local LFS objectstc-index-lfs-objects-file-store

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

1 files changed, 0 insertions, 19 deletions

diff --git a/spec/models/lfs_object_spec.rb b/spec/models/lfs_object_spec.rb
index 6e35511e848..911f85d7b28 100644
--- a/spec/models/lfs_object_spec.rb
+++ b/spec/models/lfs_object_spec.rb
@@ -2,12 +2,6 @@ require 'spec_helper'
 
 describe LfsObject do
   describe '#local_store?' do
-    it 'returns true when file_store is nil' do
-      subject.file_store = nil
-
-      expect(subject.local_store?).to eq true
-    end
-
     it 'returns true when file_store is equal to LfsObjectUploader::Store::LOCAL' do
       subject.file_store = LfsObjectUploader::Store::LOCAL
 
@@ -83,19 +77,6 @@ describe LfsObject do
     describe 'file is being stored' do
       let(:lfs_object) { create(:lfs_object, :with_file) }
 
-      context 'when object has nil store' do
-        before do
-          lfs_object.update_column(:file_store, nil)
-          lfs_object.reload
-        end
-
-        it 'is stored locally' do
-          expect(lfs_object.file_store).to be(nil)
-          expect(lfs_object.file).to be_file_storage
-          expect(lfs_object.file.object_store).to eq(ObjectStorage::Store::LOCAL)
-        end
-      end
-
       context 'when existing object has local store' do
         it 'is stored locally' do
           expect(lfs_object.file_store).to be(ObjectStorage::Store::LOCAL)