In-band upgrades: vote on exit-at-block-height

[jcnelson]

Create a designated smart contract that lets miners and STX-holders vote on a exit-at-block-height value that, when chosen, will cause the Stacks chain to cease processing blocks at that height (plus a hard-coded grace period). This is to allow the network stakeholders to vote to execute a network upgrade.

This is not a consensus-critical issue -- it can be deployed today if we wanted it, and it can be implemented after 2.1. We would need to update the Stacks block-processing logic so that the node looks at the state of this contract each time it considers a block, and if this contract indicates that an exit height has been chosen, the node refuses to process subsequent blocks (and/or exits).

[jcnelson]

This is fully discussed here: #2687