Vladimir, if `mapAndCombine` and `migrateIntermediateKeysAndValues` need different defaults, then perhaps they should be configurable separately?

Besides, the default in StateTransferConfigurationBuilder is actually 10000 (which is kind of big, even for state transfer). The "backup default" here are only used if the user explicitly configures a chunk size of 0. If we only allowed `chunkSize > 0` in StateTransferConfigurationBuilder.validate(), then we wouldn't need any "backup default" here. (Sorry about the confusion earlier, when I said we should disallow negative values, I meant to forbid 0 as well.)