workers should check eval token before submitting Ack/Nack

[tgross]

If a leader election happens while a scheduler worker is processing an eval, when it tries to submit the plan to the new leader the leader will reject these with the error message "evaluation token does not match". Field experience has shown this error message is confusing. Workers should be able to verify that the token is valid before sending the ack/nack. This will reduce useless RPC traffic during the expensive leadership transition and more importantly reduce operator confusion.

cc @schmichael @davemay99 from our conversation around #11638