multi: add leader check to the healthcheck monitor

This commit extends our healtcheck with an optional leader check. This
is to ensure that given network partition or other cluster wide failure
we act as soon as possible to avoid a split-brain situation where a new
leader is elected but we still hold onto our etcd client.

config.go

@@ -169,6 +169,17 @@ const (
 	defaultRSBackoff  = time.Second * 30
 	defaultRSAttempts = 1
 
+	// Set defaults for a health check which ensures that the leader
+	// election is functioning correctly. Although this check is off by
+	// default (as etcd leader election is only used in a clustered setup),
+	// we still set the default values so that the health check can be
+	// easily enabled with sane defaults. Note that by default we only run
+	// this check once, as it is critical for the node's operation.
+	defaultLeaderCheckInterval = time.Minute
+	defaultLeaderCheckTimeout  = time.Second * 5
+	defaultLeaderCheckBackoff  = time.Second * 5
+	defaultLeaderCheckAttempts = 1
+
 	// defaultRemoteMaxHtlcs specifies the default limit for maximum
 	// concurrent HTLCs the remote party may add to commitment transactions.
 	// This value can be overridden with --default-remote-max-htlcs.
@@ -672,6 +683,12 @@ func DefaultConfig() Config {
 				Attempts: defaultRSAttempts,
 				Backoff:  defaultRSBackoff,
 			},
+			LeaderCheck: &lncfg.CheckConfig{
+				Interval: defaultLeaderCheckInterval,
+				Timeout:  defaultLeaderCheckTimeout,
+				Attempts: defaultLeaderCheckAttempts,
+				Backoff:  defaultLeaderCheckBackoff,
+			},
 		},
 		Gossip: &lncfg.Gossip{
 			MaxChannelUpdateBurst: discovery.DefaultMaxChannelUpdateBurst,