Put to-enqueue tasks on the request-local context [#7]

This probably should've happened sooner...the list of tasks to enqueue
needs to go away after each request, or tasks will be re-enqueued at the
end of subsequent requests.
If any task fails queueing, revoke its neighbors [#7]

Since we are about to roll back, the successfully-enqueued tasks may
have references to DB keys that won't exist.