Disabling feature flag doesn't disable feature flags still in client cache

[timgl]

Bug description

Disabling feature flags doesn't actually cause the JS SDK to return false for that feature flag if it was already seen by user

How to reproduce

1. Create feature flag and enable it
2. Refresh page, see that posthog.isFeatureEnabled('query-websocket') is true
3. Disable feature flag
4. Refresh page. query-websocket doesn't show up in /decide anymore, but posthog.isFeatureEnabled('query-websocket') still returns true

Additional context

[havenbarnes]

The SDK doesn't overwrite local cache flags if the flag is suddenly missing from /decide. Sounds like we either need to return them from /decide or diff latest /decide response with the local cache flag keys