Currently, the oplog-tracking aggregation used by resharding executes on a dedicated thread. This has some drawbacks, so we would like to change it to use the Pipeline class and API instead. However, this requires a DocumentSourceMergeCursors stage to be prepended to the pipeline, and at present there are some invariants which only expect to see a postBatchResumeToken for a change stream.

Eventually we want to allow the ARM to handle either kind of postBatchResumeToken. However, this will require further engineering effort on the mongoS side and an expansion of the work carried out under SERVER-53534 on the mongoD side. In the short-term, we should relax these constraints to allow resharding postBatchResumeTokens.