Optimistic token streaming pattern

Building on the pattern in #46, design and implement a pattern for fast token streaming that doesn't go through the DB.

So e.g.:
- optimistic token streaming using broadcast
- batch up tokens into messages
- store messages in the DB
- some machinery to join things up
- handle token loss edge cases

N.b.: this pattern also really needs #27 implement to deliver the full benefit. Which also means getting https://github.com/electric-sql/electric/pull/2544 merged.