Opportunistic batching

This KEP proposes an opportunistic batching mechanism in the scheduler to improve performance of scheduling many compatible pods at once, and to begin building the infrastructure required for gang scheduling. To implement this mechanism we propose the following additions: - Pod scheduling signature: A signature that captures the properties of a pod that impact scoring and feasibility. - Batching mechanism: A mechanism to reuse the scheduling output from one pod to provide node hints for multiple subsequent pods with matching scheduling signatures. - Opportunistic batching: Transparent inclusion of the batching mechanism in the scheduler to improve the performance of targeted workloads that could benefit from it. - Rescoring: An extension to the batching mechanism that removes the one-pod-per-node limitation: when the last chosen node is still feasible, it is rescored in-place rather than flushing the batch.