message: [onchaindiscovery] mark a started service as started
  Closes:
  Approved by: davidiw
diff: @@ -74,9 +74,11 @@ impl OnchainDiscoveryBuilder {
  }
  }
  - /// Starts the provided onchain_discovery_service and onchain_discovery.
  - pub fn start(self, executor: &Handle) {
  + /// Starts the provided onchain_discovery_service and onchain_discovery. Should be called at
  + /// most once.
  + pub fn start(mut self, executor: &Handle) {
  assert!(!self.started);
  + self.started = true;
  executor.spawn(self.onchain_discovery_service.start());
  executor.spawn(self.onchain_discovery.start());
  }
  