Improve session cleanup (#18123)

- We move logging statements to the upper layer.
Store functions are low-level methods and should return error
upwards rather than logging.
- Used IN instead of any (array ()) which is equivalent.
- Made the delay to be of type time.Duration and un-exported it.
- Unexported the batch size constant.

```release-note
NONE
```

app/server.go

@@ -1511,11 +1511,15 @@ func doCommandWebhookCleanup(s *Server) {
 }
 
 const (
-	SessionsCleanupBatchSize = 1000
+	sessionsCleanupBatchSize = 1000
 )
 
 func doSessionCleanup(s *Server) {
-	s.Store.Session().Cleanup(model.GetMillis(), SessionsCleanupBatchSize)
+	mlog.Debug("Cleaning up session store.")
+	err := s.Store.Session().Cleanup(model.GetMillis(), sessionsCleanupBatchSize)
+	if err != nil {
+		mlog.Warn("Error while cleaning up sessions", mlog.Err(err))
+	}
 }
 
 func doCheckAdminSupportStatus(a *App, c *request.Context) {