MI-05 Unavailability Through Faulty Error Handling

[xoloki]

When a signer runs the method dkg_private_shares() upon receiving private share, the execution will run the line

      self.dkg_private_shares
           .insert(src_signer_id, dkg_private_shares.clone());

before attempting decryption so that if decryption fails, the hashmap will still contain the value.

[xoloki]

In order to address ME-03 (#72), we need to store the private shares regardless of whether they decrypt properly or fail to verify against the public polynomial of the sending party. That way duplicate messages can be handled quickly and without any expensive math operations.

These will be cleaned up after DKG completes as part of HI-01 (#68).