[DSS] Move downloading logic from Client Actor to StateSync Actor

[VanBarbascu]

Tasks

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1. Start an actor for each shard we want to sync and message them when we need to start the sync.
2. Handle sync requests from client actor
3. Signal back to the client actor when the sync is done
4. Message: send getParts* from peers. Initial algorithm will be to pick the parts at random
5. Message: Handle partResponse(Header|Part)