Core should retrieve and save documents atomically

[laserval]

When receiving a document from a stage, Core will do two database operations:
https://github.com/Findwise/Hydra/blob/master/core/src/main/java/com/findwise/hydra/net/MarkHandler.java#L65

https://github.com/Findwise/Hydra/blob/master/core/src/main/java/com/findwise/hydra/net/MarkHandler.java#L74

https://github.com/Findwise/Hydra/blob/master/core/src/main/java/com/findwise/hydra/net/MarkHandler.java#L108

This could mean that a document is available for the getDocumentById() call, but is missing when one of the markXXXXX() methods are called.