Switch to native identifier strategy even when using the new identifier generators

Currently, if the hibernate.id.new_generator_mappings is set to false, @GeneratedValue(strategy = GenerationType.AUTO) is mapped to native. If this property is true (which is the deafult value in 5.x), the @GeneratedValue(strategy = GenerationType.AUTO) is always mapped to SequenceStyleGenerator.

For this reason, on any database that does not support sequences natively (e.g. MySQL) we are going to use the TABLE generator instead of IDENTITY.

However, TABLE generator, although more portable, uses a separate transaction every time a value is being fetched from the database. In fact, even if the IDENTITY disables JDBC batch updates and the TABLE generator uses the pooled optimizer, the IDENTITY still scales better.