When an evaluation is acknowledged by a scheduler, the resulting plan is
guaranteed to cover up to the `waitIndex` set by the worker based on the most
recent evaluation for that job in the state store. At that point, we no longer
need to retain blocked evaluations in the broker that are older than that index.

Move all but the highest priority / highest `ModifyIndex` blocked eval into a
canceled set. When the `Eval.Ack` RPC returns from the eval broker it will
retrieve a batch of canceable evals to write to raft. This paces the
cancelations limited by how frequently the schedulers are acknowledging evals;
this should reduce the risk of cancelations from overwhelming raft relative to
scheduler progress. In order to avoid straggling batches when the cluster is
quiet, we also include a periodic sweep through the cancelable list.