* Parallelize `ec.rebuild` operations per affected volume.
* node.freeEcSlot >= slotsNeeded
* variable names, help messages,
* Protected the read operation with the same mutex
* accurate error message
* fix broken test
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Co-authored-by: chrislu <chris.lu@gmail.com>
Co-authored-by: Chris Lu <chrislusf@users.noreply.github.com>