Performance Tweaks

• Additional cleanup spurred by having temporarily moved each Rollup's instance eval evaluator variable to be non-final, which in addition to being bothersome exposed some hand-off issues between the batch class version of Rollup and its queueable inner class. This also revealed a few spots where the RollupControl__mdt was being fetched twice for no reason.
• Introduced an explanatory comment on the Database.executeBatch section of Rollup's runCalc method, as a newcomer might find themselves ... wondering ... why we were re-initializing the class 😅🔍
• small heap size improvement in RollupEvaluator (cc @ssk42, in the event you had broken ground there, but this shouldn't be a merge-conflict scenario of any kind); holdover from #91; empty object pattern (AlwaysTrueEvaluator) should always be implemented as a singleton