HotShot node able to run in "viewing-only" mode without a key in the stake table

[elliedavidson]

For the purposes of applications building on top of HotShot, a HotShot node should be able to function in a viewing-only mode if the node has either 1. zero stake, or 2. no key in the stake table. The node should still process all proposals and other messages, but will not vote in the network. The node can still determine decided blocks, however, by watching the QCs that come through.