[Sheepdog] Add more hash strategy

[HaiTing Yao]

Hi,

I read codes about hash now and tried to do something. I need some comments
to do it properly.

Strategy 1 (We used now)

Random tokens per node

Partition by random token. The partition token is produced by hash function
with node IP/port.

Strategy 2

 Random tokens per node

Equal sized partitions. The hash space is divided into Q equally sized
partitions. Q is the number of nodes or vnodes.

Strategy 3

 Random tokens per node

Equal sized partitions. When there is node join/leaving, the equal size may
be broken to minimize the data movement.

For strategy 3, need write hash space to disk. I think I can use the object
to record the infomation.

Do we need add the strategy 2/3, or there is better strategies?

Thanks
Haiti
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