Make priming run only on workspace reload

[lnicola]

Seems that our priming caching infra is indeed at least confusing: #9704 (comment)

@jonas-schievink I wonder if you see some quick wins there? One reasonable option might be to only run cache priming on configuration change (that is, when we go non-quiescent -> quiescent). That's a bit tricky to code, because priming might get interrupted by edits, and we should re-run it in that case, but we still want to not re-run it when it completes.

Another option is to make salsa internally-parallel and just get rid of cache priming :o)

Originally posted by @matklad in #9564 (comment)

[lnicola]

Basically we try to do it on every keypress. It's not really expensive in itself (just some salsa cache hits), but we spam the client with hundreds of messages. Code copes fine, but other editors like Sublime Text don't.