Cache landscape objects

[lotzk]

The most recently used landscape objects should be held in a cache to avoid I/O. As mentioned in #38, landscape objects will be persisted as json-api strings, identified by unique (numeric) id. We can utilize Redis as an in-memory-cache for those string.

[Alexander-Krause-Glau]

At the moment I am experimenting with a Redis cache to see if our stringified Mongo-DB approach is really so much slower.