**Summary:**

- Introduces the capability to perform batch insert operations with duplicated entries without throwing a unique constraint violation exception. This is achieved by adding a new prepared statement to a `MappedEntity` when it is loaded. This statement will upsert the duplicate entries and will be implemented to each engine accordingly. This way, this prepared statement (upsert) will be used in the batch processing associated with the entity when adding and flushing the entries, but using the upserting any duplicated entries. This commit introduces support for the H2 and PostgreSQL engines for this operation.

- For the H2 engine, the prepared statement is the `MERGE` command which updates existing rows, and insert rows that don't exist. The documentation for this engine can be found [here](http://www.h2database.com/html/commands.html#merge_into)

```sql
MERGE INTO <TARGET_ENTITY> (cols...) VALUES(vals...)
```

- For the PostgreSQL engine, the insert statement introduces a `ON CONFLICT` clause that specifies an alternative action to raising a unique violation or exclusion constraint violation error. For the implementation, it was added the `DO UPDATE` action which will updating the columsn to the new values. It uses the values of the special table `EXCLUDED` to update it to the new values. The documentation for this engine can be found [here](https://www.postgresql.org/docs/current/sql-insert.html)

```sql
INSERT INTO <TARGET_ENTITY> (cols...) VALUES (vals...)
ON CONFLICT
DO UPDATE
SET (cols...) = EXCLUDED.(cols...)
```

**Test:**

- A new test was added to verify that a batch with duplicate entries successfully inserts only one entry and do not throw an exception.
- The test suite was run for every supported engine (H2 and PostgreSQL).

- Reverts import order to avoid boilerplate diff.

- Minor changes in the H2Engine MERGE command.
- Refactors the INSERT ON CONFLICT in teh PostgreSqlEngine class, so it updates instead of ignoring duplicate entries.
- Refactors test.

- Fix method name.

- Fix tests: Adds `assumeTrue` so it ignores the engines for two tests where the upsert prepared statement is not implemented.

- Fix tests: Adds cockroach db to exception as it extends the Postgres implementation.

- Fix tests: Fixes the h2 assumption as it requires more than one h2 engine.

- Addresses comments.