Phase 3 (Stream cc-ing)
Streams which share prefixes are automatically logged to when the prefix is logged to.
On the mobile apps, it's trivial to generate the "cc map" from an array of stream names once the stream configuration is downloaded at the start of the session and is kept in memory until the app is closed (e.g. https://github.com/wikimedia/wikipedia-ios/pull/3603/files#diff-090a2618a1a359b3ab62798303960325R362-R374)
On MediaWiki, generating the map shouldn't be done on every pageload and should be taken care of by EventStreamConfig extension: T256169