Rate-limit verify API requests

[makew0rld]

I'm developing a web app that does a lot of OpenTimestamps.verify calls to display information in the UI. This works, except there is currently an issue where blockstream.info will respond with 429 rather than the API response when too many requests are made. It would be helpful if the OpenTimestamps lib had rate-limiting and retries built in to avoid this issue.

[petertodd]

Have you considered just putting the verify calls behind a rate limiter? I'm not the maintainer of this library. But maybe that'll help.