Initial stubs for resharding driver

[timvisee]

Tracked in: #4213

Lay out the initial stubs for the logic driving the whole resharding process.

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[ffuugoo]

I feel like we don't neccessary need ReshardTask. ReshardKey + ShardHolder::resharding_state should be enough, but LGTM otherwise.

[timvisee]

I feel like we don't neccessary need ReshardTask. ReshardKey + ShardHolder::resharding_state should be enough, but LGTM otherwise.

You might be right. My idea was that we might add additional information such as the desired shard distribution.

I'll remove it for now. We can add it back later once we think it'll be useful again.