Use iterators for reputation contract

[AnnaShaleva]

Methods Get and GetByID restrict the maximum number of DataAuditResult with 2047 items (per epoch per node).
It may be critical for Get and GetByID returning an array of audit results for a certain epoch made by a single IR node in case of a large amount of audit results, so we need to return iterator from these methods.

IIUC, ListByEpoch is not affected and may be kept as is because it returns IRNodesCount results at max (hardly ever we have the number of IR nodes more than 2047).

Ref. #304.