Consider queue-like system instead of pollers

[shanefontaine]

Important

Partial dependency on the outcome of the DB ticket. If relational DB is chosen, poller is likely replaced by indexed DB.

Do we want to replace pollers with queue-like system? Instead of polling for incompletes (sync watcher), unbonded/relayed/etc. transfers/transferRoots, can they live in queues with explicit states?

• Ideally this lives locally to avoid introducing a 3rd party queue/cache. Must handle server restarts though.
• Does this add too much complexity such that it is not worth it & polling is more ideal?