want better support for minimum versions #1754

If a server wants to retire old API versions, it's important for it to crisply reject requests for versions older than what it currently supports. This involves two pieces:

• ClientSpecifiesVersionInHeader should support an optional min version, analogous to the max version. It would do the check to reject requests with versions that are too old.
• api_versions! should define a constant for the oldest supported release so that consumers can provide it to ClientSpecifiesVersionInHeader