* feat(qcow2): improve generation speed
the idea is to build a bit index of the clusters to store them efficiently instead
of going through all the possible blocks 3 times at different phases of the generation.
The memory consumption is 2MB/TB of disk
this index generation can still be quite long , so we use setImmediate from
time to time to le the rest of the nodejs event loop run
also make this index phase abortable
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