-Only used when experimental downsampling is enabled
-Persists read iterators across shards, creating a new set only when we've exhausted
the current BAM file region(s). This prevents the engine from revisiting regions discarded
by the downsamplers / filters, as could happen in the old implementation.
-SAMDataSource no longer tracks low-level file positions in experimental mode. Can strip
out all related code when the engine fork is collapsed.
-Defensive implementation that assumes BAM file regions coming out of the BAM Schedule
can overlap; should be able to improve performance if we can prove they cannot possibly
overlap.
-Tests a bit on the extreme side (~8 minute runtime) for now; will scale these back
once confidence in the code is gained
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