vault: ensure that token revocation is idempotent
vault: ensure that token revocation is idempotent

This ensures that token revocation is idempotent and can handle when
tokens are revoked out of band.

Idempotency is important to handle some transient failures and retries.
Consider when a single token of a batch fails to be revoked, nomad would
retry revoking the entire batch; tokens already revoked should be
gracefully handled, otherwise, nomad may retry revoking the same
tokens forever.

hashicorp/nomad,burdandrei/nomad,hashicorp/nomad,hashicorp/nomad,burdandrei/nomad,burdandrei/nomad,burdandrei/nomad,dvusboy/nomad,dvusboy/nomad,hashicorp/nomad,hashicorp/nomad,burdandrei/nomad,dvusboy/nomad,burdandrei/nomad,dvusboy/nomad,dvusboy/nomad,burdandrei/nomad,dvusboy/nomad,hashicorp/nomad,dvusboy/nomad