Index agent's kernel registry by backend-dependent key

[sanxiyn]

Currently, agent's kernel registry is indexed by kernel_id, which is the backend-independent key. Backends are not aware of it, for example, Docker backend only knows about container_id of its containers.

Sometimes we need to look up kernel by backend-dependent key. Currently, we do this by creating a temporary index by iterating over entire kernel registry. kernel_id_map: Dict[ContainerId, KernelId] in stats.py is an example. Instead, we should make this index permanent and update it whenever kernel registry is updated.