refactor: delete unused Driver messages and joinedIds fields

[arbazkhan971]

Summary

Deletes the unused messages and joinedIds fields from Driver and IDriver.

messages was never assigned, so it stayed undefined. Subscription de-duplication already lives on the Socket, keyed by stream name. joinedIds was initialized to [] and never written or read. Both were public, so they looked like real driver state.

Test plan

• npx tsc --noEmit
• npm run lint
• npx jest lib/drivers consumer (313 tests)

Fixes #379

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