Delegation: Store latest acked nonce

[cam-schultz]

Context and scope
We store the validator nonce used to start and end a delegation in the calls to initializeRegisterDelegator and initializeEndDelegation. In the complete step, we verify that the received nonce from the verified Warp message is greater than or equal to the stored nonce.

Rather than requiring a Warp message be verified when completing delegation registration/removal, we could track the latest received nonce in the contract's state, and skip the Warp message requirement if we've already received a higher nonce than the stored delegator nonce.

Open questions
We should quantify the gas savings this could yield, since we'd be increasing the amount of state stored in the contract.