PPLNS with job declaration

Protocol Design
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I slightly changed the share definition share-accounting-ext/extension.md at 8cf48767de3a477ab2b11ada8fb3f05c19cce758 · demand-open-source/share-accounting-ext · GitHub

  1. Remove merkle_path from the share, the field is redundant cause we can derive it from the transactions in the job, later in the verification procedure.
  2. Add share_index: the index of the share in the slice. This solve the issue raised above by @marathon-gary (there is no way for a miner the check if the share sent by the pool are the ones required)