Stream iterators account for unordered data

[owen-d]

This fixes a bug where ingester stream iterators assumed their chunks maintained increasing inter chunk order. This is not guaranteed now that unordered writes are available.

This is a followup after closing #4101. Here, we take a much simpler route, adding inter-chunk ordering calculations while iterating through chunks, but importantly do not incur additional slice allocations doing so.

I spent some time attempting to build centralized logic for these calculations in order to reuse it in a few places with duplicate logic, but I was unable to find a working solution that didn't incur slice allocations which I want to avoid on this code path. I do, however, want to revisit these issues post the introduction of generics in go (or if someone sees something I did not).

ref #1544

[cyriltovena]

LGTM