PG to PG: Add ORDER BY in MERGE to handle non-PK unique columns (#4169)

## Problem statement
Consider a Postgres table such with `id` PRIMARY KEY and `email` TEXT
with a UNIQUE constraint on it. Let's say there is a mirror of this
table to another PG database, and the following changes are made on
source:

```sql
DELETE FROM table WHERE id = 1 (where email = 'alice@example.com')
INSERT INTO table (id, email) VALUES (2, 'alice@example.com')
```

This can result in a unique constraint violation error in normalize:
```
error executing normalize statement for table public.accounts: 
ERROR: duplicate key value violates unique constraint
```

This is because MERGE in Postgres processes rows in non-deterministic
fashion, therefore across PKeys we could attempt replicating the INSERT
first before DELETE (note: for rows pertaining to the same PKey this is
not possible as we rank there), resulting in us hitting a non-PK unique
constraint.

## Fix
According to [Postgres docs for
MERGE](https://www.postgresql.org/docs/current/sql-merge.html), it says:

```
The order in which rows are generated from the data source is indeterminate by default. 
A source_query can be used to specify a consistent ordering, if required, which might be needed to avoid deadlocks between concurrent transactions.
```

We are already using the `source_query` syntax in our MERGE. This PR
adds an ORDER BY to our subquery which sorts by `_peerdb_timestamp`
which is representative of WAL order, thereby not running into this
issue.

- [x] Functionally tested

Author: Amogh-Bharadwaj

flow/connectors/postgres/client.go

@@ -55,12 +55,12 @@ const (
 	ARRAY_AGG(DISTINCT _peerdb_unchanged_toast_columns) FROM %s.%s WHERE
 	_peerdb_batch_id=$1 AND _peerdb_record_type!=2 GROUP BY _peerdb_destination_table_name`
 	mergeStatementSQL = `WITH src_rank AS (
-		SELECT _peerdb_data,_peerdb_record_type,_peerdb_unchanged_toast_columns,
+		SELECT _peerdb_data,_peerdb_record_type,_peerdb_unchanged_toast_columns,_peerdb_timestamp,
 		RANK() OVER (PARTITION BY %s ORDER BY _peerdb_timestamp DESC) AS _peerdb_rank
 		FROM %s.%s WHERE _peerdb_batch_id = $1 AND _peerdb_destination_table_name=$2
 	)
 	MERGE INTO %s dst
-	USING (SELECT %s,_peerdb_record_type,_peerdb_unchanged_toast_columns FROM src_rank WHERE _peerdb_rank=1) src
+	USING (SELECT %s,_peerdb_record_type,_peerdb_unchanged_toast_columns FROM src_rank WHERE _peerdb_rank=1 ORDER BY _peerdb_timestamp) src
 	ON %s
 	WHEN NOT MATCHED AND src._peerdb_record_type!=2 THEN
 	INSERT (%s) VALUES (%s) %s