prefetcher: use a 1-sized channel for setting subscription
prefetcher: use a 1-sized channel for setting subscription

Suggested by jzila.

Issue: #23219

keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client