config_local: avoid panic when cleaning sync cache in background
config_local: avoid panic when cleaning sync cache in background

There could be a race where the cache is quickly reset multiple times.

Issue: HOTPOT-2335

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