Change private tally transaction to carry results computed offline

[mzabaluev]

To speed up validation of private vote tallying, the ledger logic should have access to the results computed offline and submitted along with the decrypt shares. It can then use the verification method added in input-output-hk/chain-libs#518 instead of the full tally computation.

To Do

• ledger and transaction changes in chain-impl-mockchain: Support offline private tally in ledger chain-libs#520
• update jormungandr to the changes: Update private tally transaction #2986