Move resharding state into shard holder

[timvisee]

Tracked in: #4213

Move the resharding state from the collection struct into the shard holder. It now lives next to the shard transfers state.

I've also added a ReshardingKey which holds the peer ID, shard ID and resharding key because we commonly use these together. This matches with the shard transfer key.

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[ffuugoo]

LGTM. This makes much more sense, tbh. 😂

I have some minor comments, but I'll just fix them in my next PR.

[timvisee]

I have some minor comments, but I'll just fix them in my next PR.

Sounds good!