Saw some strange behavior when using `INSERT ON CONFLICT` inside a transaction.

Repro steps:
* I created the table:
```
create table t (col TEXT primary key);
```

* Open two database consoles and use the following queries in them:

```
1 begin;
2 select txid_current();
3 insert into t (col) values ('test') on conflict (col) do nothing;
4 commit;
```

I ran 1 and 2 in both. It worked for both.
And then I ran query 3 in the first console. It worked.
After that I ran query 3 in the second console. It got stuck!
When I commit the tx in first console, the stuck query in second console
finishes
The transaction isolation level is 'read committed'.

--
Best Regards
Sagar Tiwari