Implement new Judgehost API

[meisterT]

Description of the problem

@ubergeek42 proposed a new Judgehost API in https://docs.google.com/document/d/1dMjPOcR2TqS7Z4jNbb5W5vVdHu43BSf79gRngEkfk5w/edit#

This will allow us to parallelize judging across multiple judgehosts, to lazily evaluate test cases after the final verdict for a submission is known or do generic tasks like preheating judgehosts or collecting debug information.

Some related issues are: #258, #215, #162, #639

[meisterT]

With some related commits in the past few days I consider this issue closed. The last big issue is #215, but that can be and is tracked separately.