Make CDF readers streaming (or otherwise read large CDF files without running out of memory)

[moldover]

Currently CDF readers use Jackson to map CVR files into memory all at once. This will not fly for any decent-sized election as @freedomcounts has already discovered we cannot tabulate a single 80k record CDF contest.

So we need some streaming solution. This is complicated by the fact that "static" top-level election definition objects must be parsed before cvrs are prased. Note that static objects probably can be read directly into memory if that's any help - they will be of a reasonable size, even for a big election. The cvr objects however will need to be streamed.

[JDziurlaj]

For JSON, it should be fairly easy to construct a CDF CVR where the static information appears before the CVRs, since JSON keys are always unordered. XML is ordered, and unfortunately not ideally. There is an issue (usnistgov/CastVoteRecords#27) to address this in a subsequent release.