Sort traces on flush to ensure consistent payloads in the backend

[mdisibio]

What this PR does:
We have noticed that with RF3, the compactors seem to be combining more traces than expected, i.e. all ingesters should be receiving identical payloads and flushing identical bytes to the backend, and other than for long-running traces, the compactors should not have to recombine much. Researching shows that the assumption that ingesters are flushing identical bytes is incorrect. It was observed that the ingesters all flushed the same data in total for a trace, but internally the batches were not in the same order. This leads to differing bytes and to be recombined by the compactor.

This PR internally sorts the traces as they are flushed by the ingesters. The sort order doesn't really matter, as long as it is consistent. Right now it sorts bottom up by span start time, then span id.

Also considered possible causes for the different batch order: It is possible that ring.DoBatch in the distributor is not issuing the batches for a given trace in the same order to each ingester. Tried sorting indexes to fix but it did not solve the issue.

Which issue(s) this PR fixes:
n/a

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]