### What changes were proposed in this pull request?
SlotsAllocator should select disks randomly in RoundRobin mode
### Why are the changes needed?
The current round robin selection mechanism is to select the first disk of each worker first, then the second disk of each worker, and finally the third disk. This can easily cause disk storage space skew. We should select disks randomly instead of selecting the first disk first.
### Does this PR introduce _any_ user-facing change?
No.
### How was this patch tested?
Existing UTs.
Closes#3275 from leixm/CELEBORN-2008.
Authored-by: Xianming Lei <xianming.lei@shopee.com>
Signed-off-by: Wang, Fei <fwang12@ebay.com>
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