When Vault token expires and task is restarted, emit `TaskRestartSignal`
similar to v0.8.7

Client.Shutdown now blocks until all AllocRunners and TaskRunners have
exited their Run loops. Tasks are left running.

Now passing the TaskDir struct to prestart hooks instead of just the
root task dir itself as dispatch needs local/.

client: fingerprint driver plugins

Updates are applied asynchronously but sequentially