Add init flag for incentives address

[futurechimp]

We have been paying incentives manually thus far, with registration through a web form. Let's move it to the mixnode, so that each node nominates a payment address for its rewards.

So we'll add an optional --incentives-address abcdef123456 flag which nominates a payment address string. Alexis says he's able to distinguish between his existing testnet token payment types based on address format so this can just be a simple string.

We will need to add this field to the Mixnode entity on the validator side as well so it can be submitted through the API and stored on the blockchain.