Require explicit Consul key

[benbjohnson]

This pull request removes the default Consul key as it is too easy to accidentally connect 2 clusters together if you're using the same Consul. This can have some very unexpected results.

Now, users will need to explicitly state the key they want to use for the cluster:

consul:
  url: "${FLY_CONSUL_URL}"
  key: "litefs/${FLY_APP_NAME}"
  advertise-url: "http://${HOSTNAME}.vm.${FLY_APP_NAME}.internal:20202"

Previously, the key was "litefs/primary" but it can be any value that is unique to your cluster (e.g. "production", "staging", etc). On Fly.io, Consul usage is scoped to an organization so it only need to be a unique key within your org.