Bugfix: Fix crashing in postgres-backed storage (microsoft#1000)

# Pull Request

## Title

In PostgreSQL, attempting to insert a record with conflicting keys fails
(a benign action in MLOS), fails the entire transaction thus preventing
the update of other tables. Splitting the update into two independent
transactions solves the problem.
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## Description

Update tables `trial_status` and `trial` in separate transactions
(that's OK for MLOS) to make sure PG won't fail the update of `trial` if
there is a duplicate record in `trial_status`.

- **Issue link**: Closes microsoft#999 

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## Type of Change

- 🛠️ Bug fix

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## Testing

Run TQP benchmark with PostgreSQL storage.

Author: motus

mlos_bench/mlos_bench/storage/sql/trial.py

@@ -116,6 +116,9 @@ def update(
         metrics = super().update(status, timestamp, metrics)
         with self._engine.begin() as conn:
             self._update_status(conn, status, timestamp)
+        # Use a separate transaction to avoid issues with PostgreSQL's duplicate key
+        # constraint handling. (See Issue #999).
+        with self._engine.begin() as conn:
             try:
                 if status.is_completed():
                     # Final update of the status and ts_end: