The aggressive RETRY_INTERVAL in PaymentController consumes enormous CPU resources

Created by: crwood

https://github.com/PrivateStorageio/ZKAPAuthorizer/commit/cb534cc1d465a5ba8b414faa5a9d41506e59beda reduced the retry timer used by controller.PaymentController._schedule_retries to 0 seconds (while a later commit, https://github.com/PrivateStorageio/ZKAPAuthorizer/commit/6343ce3885467e63bf1baf52d88e48c1e5eff85f, increased this value to 1 milliseconds) . On My Machine, this very short value causes otherwise idle tahoe processes to sustain 100% CPU usage indefinitely.

Increasing RETRY_INTERVAL to 1000 milliseconds brings CPU-usage levels back down to less-knee-scorching single-digit percentages, though I'm unsure whether this constitutes a reasonable adjusted value (and/or whether changing it would have any adverse effects on other parts of the system -- beyond introducing an extra-second wait-time at the beginning of redemption-attempts). I'm happy to submit a PR for this, in any case; please advise what an appropriate value might be.