Bring sql driver on par with benchmark implementation

[breml]

In the benchmark, there have been made some changes related to the sql driver, that have never been fed back to the actual sql driver (watermill-sql).

For PostgreSQL driver:

• Use of PRIMARY KEY on field offset (https://github.com/ThreeDotsLabs/watermill-benchmark/blob/master/pkg/pubsub.go#L355 versus https://github.com/ThreeDotsLabs/watermill-sql/blob/master/pkg/sql/schema_adapter_postgresql.go#L20)
• Use of UUID data type on field uuid (https://github.com/ThreeDotsLabs/watermill-benchmark/blob/master/pkg/pubsub.go#L356 versus https://github.com/ThreeDotsLabs/watermill-sql/blob/master/pkg/sql/schema_adapter_postgresql.go#L21)
• Use of BYTEA data type on field payload (https://github.com/ThreeDotsLabs/watermill-benchmark/blob/master/pkg/pubsub.go#L358 versus https://github.com/ThreeDotsLabs/watermill-sql/blob/master/pkg/sql/schema_adapter_postgresql.go#L23)

For the MySQL driver:

• Use of BINARY(16) data type on field uuid (https://github.com/ThreeDotsLabs/watermill-benchmark/blob/master/pkg/pubsub.go#L338 versus https://github.com/ThreeDotsLabs/watermill-sql/blob/master/pkg/sql/schema_adapter_mysql.go#L46)
• Use of BLOB data type on field payload (https://github.com/ThreeDotsLabs/watermill-benchmark/blob/master/pkg/pubsub.go#L340 versus https://github.com/ThreeDotsLabs/watermill-sql/blob/master/pkg/sql/schema_adapter_mysql.go#L48)