support overwrite with load_labels_from_file

My customers report:
```sql
1205=# select load_labels_from_file('owg', 'Person', 'overwrite_load/file2.csv');
ERROR: Cannot insert duplicate vertex id: 957014890901079
HINT: Entry id 112589960769111 is already used
```

They expect overwrite the existing vertex, which is simliar to INSERT ... ON CONFLICT SET ...

Although load into a temp table and using insert .. on conflict is viable, but customer can't accept it for complex operation.

So, I'll try to implement it check existence first and then update the conflicted tuple just like INSERT .. ON CONFLICT. Is it a reasonable approach?