When file header extent size grows, data section needs to be moved to a higher offset.
This PR moves data section by dividing the data amount to be moved among all ranks.
If the divided amount is larger than MOVE_UNIT (set to 64 MiB in this PR), then the
movement is done in multiple rounds, one chunk at a time. The chunk size has an
upper bound of MOVE_UNIT.
When file header extent size grows, data section needs to be moved to a higher offset. This PR moves data section by dividing the data amount to be moved among all ranks. If the divided amount is larger than MOVE_UNIT (set to 64 MiB in this PR), then the movement is done in multiple rounds, one chunk at a time. The chunk size has an upper bound of MOVE_UNIT.