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
```

store/sqlstore/session_store.go

@@ -12,12 +12,11 @@ import (
 	"github.com/pkg/errors"
 
 	"github.com/mattermost/mattermost-server/v6/model"
-	"github.com/mattermost/mattermost-server/v6/shared/mlog"
 	"github.com/mattermost/mattermost-server/v6/store"
 )
 
 const (
-	SessionsCleanupDelayMilliseconds = 100
+	sessionsCleanupDelay = 100 * time.Millisecond
 )
 
 type SqlSessionStore struct {
@@ -275,12 +274,10 @@ func (me SqlSessionStore) AnalyticsSessionCount() (int64, error) {
 	return count, nil
 }
 
-func (me SqlSessionStore) Cleanup(expiryTime int64, batchSize int64) {
-	mlog.Debug("Cleaning up session store.")
-
+func (me SqlSessionStore) Cleanup(expiryTime int64, batchSize int64) error {
 	var query string
 	if me.DriverName() == model.DatabaseDriverPostgres {
-		query = "DELETE FROM Sessions WHERE Id = any (array (SELECT Id FROM Sessions WHERE ExpiresAt != 0 AND :ExpiresAt > ExpiresAt LIMIT :Limit))"
+		query = "DELETE FROM Sessions WHERE Id IN (SELECT Id FROM Sessions WHERE ExpiresAt != 0 AND :ExpiresAt > ExpiresAt LIMIT :Limit)"
 	} else {
 		query = "DELETE FROM Sessions WHERE ExpiresAt != 0 AND :ExpiresAt > ExpiresAt LIMIT :Limit"
 	}
@@ -290,16 +287,16 @@ func (me SqlSessionStore) Cleanup(expiryTime int64, batchSize int64) {
 	for rowsAffected > 0 {
 		sqlResult, err := me.GetMaster().Exec(query, map[string]interface{}{"ExpiresAt": expiryTime, "Limit": batchSize})
 		if err != nil {
-			mlog.Error("Unable to cleanup session store.", mlog.Err(err))
-			return
+			return errors.Wrap(err, "unable to delete sessions")
 		}
 		var rowErr error
 		rowsAffected, rowErr = sqlResult.RowsAffected()
 		if rowErr != nil {
-			mlog.Error("Unable to cleanup session store.", mlog.Err(err))
-			return
+			return errors.Wrap(err, "unable to delete sessions")
 		}
 
-		time.Sleep(SessionsCleanupDelayMilliseconds * time.Millisecond)
+		time.Sleep(sessionsCleanupDelay)
 	}
+
+	return nil
 }