Improve equals method of JpaEntity

Implement MongoDB support

• Implement backward compatibility with Spring Boot 1.5+ / 2.0+, Spring 4.3+ / 5.0+
• Rename method delete of CrudRepo to del to provide compatibility with Spring Boot 1.5+ / Spring 4.3+

• Implement callback methods onCreate and onUpdate in AbstractCrudService;
• Implement callback methods onCreateEvent, onUpdateEvent and onDeleteEvent in AbstractCrudService to give ability to create custom events;
• remove CreateEntityEvent, UpdateEntityEvent and DeleteEntityEvent;
• remove getToUpdateById and getToDeleteById from base CrudRepo interface;

• Move event classes to model module;
• Add an example of event handling in demo module