Rewrite ScoringService

ScoringService contained some "oddities". For example: at start it
loaded all submissions from the database, it scored them at a rate of
four each half a second, etc. These were remnants of the ScoreType
Relative, the blocking async loop and the RWS forwarding.

This commit removes all of them, making larger use of the features
provided by gevent. The performance boost is huge.

The RPC API has been left the same, only the internals were changed.

ScoringService has been made contest-agnostic, i.e. it now handles
submissions of all contests simultaneously.