Optimize Schema Migrations for simple use cases

[mephinet]

While there might be optimizations possible on how fields are stored in the graph, please investigate in the following:

• the use case "one field is added to a schema" is probably the most common. In this case, no schema migration shall be necessary. In this case, the ES also doesn't need to be reindexed, and only the mapping needs to be updated.
• the use case "one field was removed from a schema" is also pretty likely. In this case, it might also be possible to short-circuit the migration.

[philippguertler]

See also https://github.com/gentics/mesh-incubator/issues/210