Support for timestamps, other per-block metadata

[amiller]

Ethereum blocks include a lot of additional information besides just the transactions. Namely, timestamps, votes to raise or lower the per-transaction limit, etc. In consortium applications we may wish to have new node introductions/dismissals in as well.

One way to do this would be to include estimates of metadata in the ACS agreed-upon values, concatenated with the threshold-encrypted transactions. Then the protocol would guarantee that we collect estimates from at least f+1 honest parties, in which case we could do something like take the "median" value.