store: use ToSQL (#28681)

- Improve some code to use ToSQL
- Fix a bug in SqlReactionStore.GetForPostSince
where OrderBy wasn't added properly. Added a test.
- Improve a method to return a slice rather than
pass a pointer to slice.

```release-note
NONE
```

server/channels/store/storetest/reaction_store.go

@@ -366,6 +366,8 @@ func testReactionGetForPostSince(t *testing.T, rctx request.CTX, ss store.Store,
 		_, err := ss.Reaction().Save(reaction)
 		require.NoError(t, err)
 
+		time.Sleep(5 * time.Millisecond)
+
 		if del > 0 {
 			_, err = ss.Reaction().Delete(reaction)
 			require.NoError(t, err)
@@ -383,6 +385,7 @@ func testReactionGetForPostSince(t *testing.T, rctx request.CTX, ss store.Store,
 		returned, err := ss.Reaction().GetForPostSince(postId, later-1, "", false)
 		require.NoError(t, err)
 		require.Len(t, returned, 2, "should've returned 2 non-deleted reactions")
+		assert.Less(t, returned[0].CreateAt, returned[1].CreateAt)
 		for _, r := range returned {
 			assert.Zero(t, r.DeleteAt, "should not have returned deleted reaction")
 		}
@@ -394,7 +397,10 @@ func testReactionGetForPostSince(t *testing.T, rctx request.CTX, ss store.Store,
 		require.NoError(t, err)
 		require.Len(t, returned, 3, "should've returned 3 reactions")
 		var count int
-		for _, r := range returned {
+		for i, r := range returned {
+			if i > 0 {
+				assert.Less(t, returned[i-1].CreateAt, returned[i].CreateAt)
+			}
 			if r.DeleteAt > 0 {
 				count++
 			}