grokmirror sleeps 5s in some places

I'm on grokmirror.git commit ceb53836b59c9464 (Don't force url to contain hostname, 2021-08-27) and I see some multiple 5s delays from grok-pull.

It's probably not a big deal for normal users, but it's also part of the public-inbox test suite, and the test which tests against grok-pull can end up taking longer than all the other tests combined.

I don't know Python well, but some sort of pipe/socket notification could be used to avoid busy waiting.

If keeping busy waits is far easier, I've found ~100ms delays acceptable for keeping overall CPU usage down (at least from other scripting languages; Python can't be that much slower).

Anyways, I'm disabling the test by default, now; so not a high priority.