added ProposalEngine.LearningFromMistakes that provides mechanism for guarding failed value configurations;
added ProposalEngine.IteratingAllDomainsLearningFromMistakes that implements filtering of failed configurations;
added currentProposalUnconditionallyAccepted to ProposalNegotiationState;
the unconditionally accepted proposals (the top-priority ones) are guarded as mistaken after being successfully reassessed by a newly higher-priority agents' rejection;
 the currentProposalUnconditionallyAccepted is not reset on `updateProposal` if the current negotiation values haven't changed;
added DomainIterator.Random, used it for default agent implementation (NegotiationControllerBuilder);