Add denormalized Eval.DequeueID RPC

[schmichael]

#5407 made me realize that thanks to #5381 there's no reason to serialize the whole Evaluation over the Dequeue RPC because the worker will wait until the Eval is available locally before processing it. #5411 makes retrieving the Eval from the state store even easier.

Proposal

A new RPC could be added - DequeueID - that replaces the *structs.Evaluation in the response with just an EvalID string. Workers would then retrieve the Eval by EvalID after waiting on the WaitIndex.

Performance

struct.Evaluation is small, so unless ClassEligibility or FailedTGAllocs are large, the performance impact may be negligible.