Improve documentation, remove dead code plus some minor refactorings

This improves our code-to-comments ratio by a lot.

On the refactorings:

- Simplify the cluster join mode to just a single protobuf message -
  a node can either join an existing cluster or bootstrap a new one.
  All of the node-level setup like hostname and trust backend is done
  using the setup call, since those are identical for both cases.

- We don't need a node name separate from the hostname. Ideally, we would
  get rid of IP addresses for etcd as well.

- Google API design guidelines suggest the `List` term (vs. `Get`).

- Add username to comments for consistency. I think the names provide
  useful context, but git blame is a thing. What do you think?

- Fixed or silenced some ignored error checks in preparation of using
  an errcheck linter. Especially during early boot, many errors are
  obviously not recoverable, but logging them can provide useful debugging info.

- Split up the common package into smaller subpackages.

- Remove the audit package (this will be a separate service that probably
  uses it own database, rather than etcd).

- Move storage constants to storage package.

- Remove the unused KV type.

I also added a bunch of TODO comments with discussion points.
Added both of you as blocking reviewers - please comment if I
misunderstood any of your code.

Test Plan: Everything compiles and scripts:launch works (for whatever that's worth).

X-Origin-Diff: phab/D235
GitOrigin-RevId: 922fec5076e8d683e1138f26d2cb490de64a9777
diff --git a/core/internal/common/setup.go b/core/internal/common/setup.go
index 331d29a..1124d27 100644
--- a/core/internal/common/setup.go
+++ b/core/internal/common/setup.go
@@ -18,20 +18,25 @@
 
 import "git.monogon.dev/source/nexantic.git/core/generated/api"
 
+// TODO(leo): merge api and node packages and get rid of this extra layer of indirection?
+
 type (
 	SetupService interface {
 		CurrentState() SmalltownState
 		GetJoinClusterToken() string
-		SetupNewCluster(name string, externalHost string) error
+		SetupNewCluster() error
 		EnterJoinClusterMode() error
-		JoinCluster(name string, clusterString string, externalHost string, certs *api.ConsensusCertificates) error
+		JoinCluster(initialCluster string, certs *api.ConsensusCertificates) error
 	}
 
 	SmalltownState string
 )
 
 const (
-	StateSetupMode       SmalltownState = "setup"
+	// Node is unprovisioned and waits for Setup to be called.
+	StateSetupMode SmalltownState = "setup"
+	// Setup() has been called, node waits for a JoinCluster or BootstrapCluster call.
 	StateClusterJoinMode SmalltownState = "join"
-	StateConfigured      SmalltownState = "configured"
+	// Node is fully provisioned.
+	StateConfigured SmalltownState = "configured"
 )