Factoring out common parts of several similar services.

ScoringService, PrintingService, ProxyService, and EvaluationService
have all a similar pattern (getting notified of object that needs an
action, and running a sweeper to catch missing object). This commit
introduce an appropriate base class, TriggeredService, that takes care
of setting up these mechanisms.

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First iteration of the full work.

ProS, PriS and SS work and are conceptually stable (I think). ES still
does not work, there are a few things to amend, and needs some
conceptual changes.

More precisely, in ES there are still a lot of code path going from
the service to the executor to the pool that need to be simplified
(not sure if I'll be able to skip some passages, but at least I need
to put the logic in only one place).