Implement sharding, other CPQ requirements

[smartin015]

Continuous Print expects an ordered list of jobs. PP is currently designed with an unordered hashtable of jobs. We need to bridge the gap.

Stern-Brocot Tree based sorting

https://begriffs.com/posts/2018-03-20-user-defined-order.html#approach-3-true-fractions

Implement this method of selecting sortable factions in golang, and use it to assign an order to jobs that get inserted. Collisions must be handled gracefully (i.e. fallback to comparing job hash, also detect race conditions and somehow get the racing shards to agree).

Peers must listen on job addition requests to keep track of the upper/lower bound of the fractional sorting value, so that new jobs can be added appropriately.

Stats and job fetching

Jobs should have a set of tags that can be queried to get certain types of jobs. Additionally, there should be RPC handlers for getting the number of jobs between two rank values.

RPC based actions

Making a request and awaiting a response leads to simpler command behaviors, especially necessary when performing sequenced actions such as sharding off part of the job queue.

Need to create an RPCBroker interface which wraps pubsub and provides this blocking call behavior, with timeouts etc.

Must also generalize the existing poll client behavior to allow multi-response requests over pubsub.