ActiveJobKey interface

Identifies a specific instance of a Job. (jobId, role) is unique within Manager; a 2-replica bundle has two ActiveJobKeys sharing jobId but with different role (e.g. "primary"/"backup").

role is the instance KEY Manager routes by. It is usually the same string as the slot the job is told in NORSK_RUNTIME_ROLE, but a migration target wears a key unique to its migration (e.g. "primary@3f9c1e2a") while its slot stays "primary", so the source and the target of one migration are addressable distinctly.

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