Optimistic token streaming pattern

[thruflo]

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 electric-sql/electric#2544 merged.