[DESIGN] auction-like priorities for servers

[justheuristic]

[for the record: this was proposed by @TimDettmers ]

Currently, hivemind-server treats all requests on a first come first served basis.
If we want to reward active participants with faster inference/training, we could change that into an auction.

Here's how client-server interaction looks like:

• server gives client its stats, the current-highest bid, and maybe some metadata for bidding, e.g. the lowest serviced bids over last T seconds
• client makes a bid - and signs it in such a way that it becomes a commitment ( see [RESEARCH] LM API merit system #6 )
• in TaskPool.iterate_minibatches, server will now generate minibatches in the order of highest bid first
• in TaskPool.priority, server will now set pool's priority based on highest bid in the pool, instead of wait time

As suggested by @GreenFatGuy , we need to think through how to deal with situations when low bids on high-demand servers won't ever be processed, and will hence take up memory on both client and server. First order solution: add absolute expiration time to each request, drop requests that hit expiration time.

[justheuristic]

currently, @GreenFatGuy investigates how best to implement that

[justheuristic]

Server-side parts implemented in #47

[justheuristic]

[next step: let users actually designate these priorities, to be added as a separate issue]