Lean on datastore queries in provider queue

[Stebalien]

At the moment, we read in the head/tail of the provider queue on start. However, it's probably easier to just repeatedly query the datastore.

That is,

1. Store provider queue entries as /provider/queue/$nanosecond-timestamp/$cid -> (or even just /provider/queue/$start-timestamp.$counter).
2. Each time we're signaled (size-one channel?) that something was enqueued, query Query{Prefix: "/provider/queue/", Order: [OrderByKey]}.

Motivation: track less state. This is really low-priority, it just may simplify this queue a bit and remove room for bugs.

[michaelavila]

Noted 👍. I'll look into this as I'm working on the remainder of the provider PRs. Thanks for bringing it up.

[michaelavila]

Closing as this has been implemented. Thanks for the suggestion!