Set traits to individual events instead of all events in a batch

The patch fixes an issue with incorrect traits setting.
Steps to reproduce an issue:
- Set some traits.
- Track an event.
- Change traits (for example, add a user email).
- Track another event. **Important**: events should be in the same batch.
- Observe batched events

**Actual result**: both events have the same traits (with last changes applied).
**Expected result**: events have individual traits in a batch.