Index agent's kernel registry by backend-dependent key

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.

JIRA Issue: BA-214