Allow clients to estimate job sizes

[talex5]

At the moment, the scheduler hard-codes time estimates for cached and non-cached jobs based on ocaml-ci's usage patterns:

https://github.com/ocurrent/ocluster/blob/6e6e2b283e57165dee484519c368c334a00a6e59/scheduler/cluster_scheduler.ml#L33-L38

We should let the client pass its own estimates.