Polling observe driver without invalidation crossbar listener

[Torgen]

Currently the polling driver queries the database at two times:

1. When a change is made in-process that might affect the results
2. Every poll interval

I want to be able to use 2 and not 1. My data changes frequently enough that I don't want to push every change to every client, and I'm willing to tolerate stale data, but AIUI setting the polling throttle to a large value can make methods slow since they wait to return until all subscriptions are updated.