Unable to load Timeseries RDD from Parquet without sorting

Hi,

If I save a TimeseriesRDD to parquet as described [in the documentation](https://ts-flint.readthedocs.io/en/latest/context.html#writing-temporary-data-to-hdfs) and then try to reread it without sorting then it will often fail with an error of the form:
`"Partitions are not sorted. The partition 0 has the first key 1541810691000000000 and the partition 1 has the first key 1541780937000000000."

Doing a bit more research- I can see you have [PR#16](https://github.com/twosigma/flint/pull/16) which I think describes the issue in more detail and says that you are working around it using custom patched version of spark. I can see that the PR is year or so old, so presume it doesn't solve the issue?

If this is the case is there any other workaround that can be used so that you can save a timeseries rdd to disk and subsequently reload it without paying the cost of a sort?

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