While copying session history in chunk migration, the sessionCatalogDestinationManager creates a CancelableOperationContext per oplog entry received from the donor. CancelableOperationContext will create a future that is kept alive until the CancellationSource of its given token is destructed. If a large number of session entries are migrated or there are a large number of migrations, this can use significant memory.

We should consider possibly create a fresh sub CancellationSource just the duration of each processed batch of oplog entries (ie create a new one for each iteration of this loop), so copying a large number of sessions during one migration won't accumulate too much memory at least until SERVER-103945 is done.