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.

lncfg/healthcheck.go

@@ -34,6 +34,8 @@ type HealthCheckConfig struct {
 	TorConnection *CheckConfig `group:"torconnection" namespace:"torconnection"`
 
 	RemoteSigner *CheckConfig `group:"remotesigner" namespace:"remotesigner"`
+
+	LeaderCheck *CheckConfig `group:"leader" namespace:"leader"`
 }
 
 // Validate checks the values configured for our health checks.