The allocrunner's `identity_hook` implements the interface for TaskStop, but
this interface is only ever called for task-level hooks. This results in a
leaked goroutine that tries to periodically renew WIs until the client shuts
down gracefully.
Add an implementation for the allocrunner's `PreKill` and `Destroy` hooks, so
that whenever an allocation is stopped or garbage collected we stop renewing its
Workload Identities. This also requires making the `Shutdown` method of `WIDMgr`
safe to call multiple times.
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