Rework schema field diff mechanism

[Jotschi]

Currently the schema diff mechanism is spread across multiple classes and thus it is very hard to keep track of needed changes.
Additionally each case has to be implemented individually.

It would be better to use a more generic approach which automatically creates the needed change information based on the diff of the REST models.

• https://github.com/gentics/mesh/blob/dev/mdm/orientdb-api/src/main/java/com/gentics/mesh/core/data/schema/handler/AbstractFieldSchemaContainerComparator.java#L94

• https://github.com/gentics/mesh/blob/dev/core/src/main/java/com/gentics/mesh/core/endpoint/schema/SchemaCrudHandler.java