"Use upstream" mode

[RCasatta]

A calendar server can operate by simply calling another upstream server for timestamping, while performing a transaction by itself if the upstream is not available or if the ots proof doesn't upgrade in a given timeframe.

This can be useful in two contexts:

• For an official calendar server to save onchain fees.
• For private entities that don't want to blindly rely on public calendars. By setting up a private calendar server with this mode, they both avoid trusting the calendar server and in the happy path avoid onchain fees.

The calendar server must expose two new parameters:

• --upstream <URL>
• --upstream-timeout N

Test harness must be written to ensure this feature is working correctly.