kvserver: add assertions for invariants around up liveness records

Now that we have cockroachdb#53842, we maintain the invariant that there always
exists a liveness record for any given node. We can now simplify our
handling of liveness records internally: where previously we had code to
handle the possibility of empty liveness records (we created a new one
on the fly), we can change them to assertions to verify that's no longer
possible.

When retrieving the liveness record from our in-memory cache,
it's possible for us to not find anything due to gossip delays. Instead
of simply giving up then, now we can read the records directly from KV
(and update our caches while in the area). This PR introduces this
mechanism through usage of `getLivenessRecordFromKV`.

Finally, as a bonus, this PR also surfaces a better error when trying to
decommission non-existent nodes. We're able to do this because now we
can always assume that a missing liveness record, as seen in the
decommission codepath, implies that the user is trying to decommission a
non-existent node.

---

We don't intend to backport this to 20.2 due to the hazard described in
\cockroachdb#54216. We want this PR to bake on master and (possibly) trip up the
assertions added above if we've missed anything. They're the only ones
checking for the invariant we've introduced around liveness records.
That invariant will be depended on for long running migrations, so
better to shake things out early.

Release note: None

pkg/cli/error.go

@@ -20,6 +20,7 @@ import (
 	"strconv"
 	"strings"
 
+	"github.com/cockroachdb/cockroach/pkg/kv/kvserver"
 	"github.com/cockroachdb/cockroach/pkg/security"
 	"github.com/cockroachdb/cockroach/pkg/server"
 	"github.com/cockroachdb/cockroach/pkg/sql/pgwire/pgcode"
@@ -359,6 +360,11 @@ func MaybeDecorateGRPCError(
 			return server.ErrClusterInitialized
 		}
 
+		// Are we trying to recommission/decommision a node that does not exist?
+		if strings.Contains(err.Error(), kvserver.ErrMissingLivenessRecord.Error()) {
+			return fmt.Errorf("node does not exist")
+		}
+
 		// Nothing we can special case, just return what we have.
 		return err
 	}

pkg/kv/kvserver/helpers_test.go

@@ -515,11 +515,10 @@ func (nl *NodeLiveness) SetDrainingInternal(
 
 func (nl *NodeLiveness) SetDecommissioningInternal(
 	ctx context.Context,
-	nodeID roachpb.NodeID,
-	liveness LivenessRecord,
+	oldLivenessRec LivenessRecord,
 	targetStatus kvserverpb.MembershipStatus,
 ) (changeCommitted bool, err error) {
-	return nl.setMembershipStatusInternal(ctx, nodeID, liveness, targetStatus)
+	return nl.setMembershipStatusInternal(ctx, oldLivenessRec, targetStatus)
 }
 
 // GetCircuitBreaker returns the circuit breaker controlling