Pursuant to the requested safety checks on the database, to ensure that
plugin code (i.e., third party code) cannot modify the database, we
implement evaluator tagging so the chain can distinguish between
consensus evaluators and third party evaluators. Also, define a new kind
of evaluator base class, third_party_evaluator, so that fees are not
charged multiple times for operations with multiple evaluators.
Next step, implement the actual safety check mechanism on the database.
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