Logic of is_timestamp_complete

[RCasatta]

According to the is_timestamp_complete function you need one BitcoinBlockHeaderAttestation to consider the timestamp complete. However it could happen than another calendar server (in PendingAttestation) has a BitcoinBlockHeaderAttestation which has an eventually lower than the first height returned.
This information will never be asked to the calendar if I have a timestamp upgraded from the other calendar.
What about having an option in upgrade to consider a timestamp complete if all PendingAttestation are served?

[petertodd]

Yup, I think that makes sense - it's not complete until you have attestations for every pending attestation.

Equally, like I mentioned in
#27, the upgrade command should always try to get new attestations even if the timestamp is complete: the calendar might be returning an even earlier timestamp than it previously did.