multi: add tor connection healthcheck

This commit adds a new health check, tor connection, to our liveness
monitor. A monitor refactor is applied to the server creation such that
the scope of health check creation is managed within one function.

lncfg/healthcheck.go

@@ -28,6 +28,8 @@ type HealthCheckConfig struct {
 	DiskCheck *DiskCheckConfig `group:"diskspace" namespace:"diskspace"`
 
 	TLSCheck *CheckConfig `group:"tls" namespace:"tls"`
+
+	TorConnection *CheckConfig `group:"torconnection" namespace:"torconnection"`
 }
 
 // Validate checks the values configured for our health checks.
@@ -50,6 +52,10 @@ func (h *HealthCheckConfig) Validate() error {
 		return errors.New("disk required ratio must be in [0:1)")
 	}
 
+	if err := h.TorConnection.validate("tor connection"); err != nil {
+		return err
+	}
+
 	return nil
 }