server: Require either cluster version v3.6 or --experimental-enable-lease-checkpoint-persist to persist lease remainingTTL

To avoid inconsistant behavior during cluster upgrade we are feature
gating persistance behind cluster version. This should ensure that
all cluster members are upgraded to v3.6 before changing behavior.

To allow backporting this fix to v3.5 we are also introducing flag
--experimental-enable-lease-checkpoint-persist that will allow for
smooth upgrade in v3.5 clusters with this feature enabled.

server/embed/config_test.go

@@ -291,6 +291,56 @@ func TestPeerURLsMapAndTokenFromSRV(t *testing.T) {
 	}
 }
 
+func TestLeaseCheckpointValidate(t *testing.T) {
+	tcs := []struct {
+		name        string
+		configFunc  func() Config
+		expectError bool
+	}{
+		{
+			name: "Default config should pass",
+			configFunc: func() Config {
+				return *NewConfig()
+			},
+		},
+		{
+			name: "Enabling checkpoint leases should pass",
+			configFunc: func() Config {
+				cfg := *NewConfig()
+				cfg.ExperimentalEnableLeaseCheckpoint = true
+				return cfg
+			},
+		},
+		{
+			name: "Enabling checkpoint leases and persist should pass",
+			configFunc: func() Config {
+				cfg := *NewConfig()
+				cfg.ExperimentalEnableLeaseCheckpoint = true
+				cfg.ExperimentalEnableLeaseCheckpointPersist = true
+				return cfg
+			},
+		},
+		{
+			name: "Enabling checkpoint leases persist without checkpointing itself should fail",
+			configFunc: func() Config {
+				cfg := *NewConfig()
+				cfg.ExperimentalEnableLeaseCheckpointPersist = true
+				return cfg
+			},
+			expectError: true,
+		},
+	}
+	for _, tc := range tcs {
+		t.Run(tc.name, func(t *testing.T) {
+			cfg := tc.configFunc()
+			err := cfg.Validate()
+			if (err != nil) != tc.expectError {
+				t.Errorf("config.Validate() = %q, expected error: %v", err, tc.expectError)
+			}
+		})
+	}
+}
+
 func TestLogRotation(t *testing.T) {
 	tests := []struct {
 		name              string