Ability to kill the Serve method

[OCharless]

Currently, the bgp server is running endlessly. It's goten shut down when the main program stops. This PR have as a purpose to supply an optional channel in order to stop gracefully the Serve method whenever it's needed.

[fujita]

Already supported:

gobgp/api/gobgp.proto

Line 36 in 815683f

rpc StopBgp(StopBgpRequest) returns(google.protobuf.Empty);

[saser]

Also note that the BgpServer type has a Stop method that will call the StopBgp API method for you:

gobgp/pkg/server/server.go

Lines 219 to 225 in 815683f

	func (s *BgpServer) Stop() {
	s.StopBgp(context.Background(), &api.StopBgpRequest{})
	if s.apiServer != nil {
	s.apiServer.grpcServer.Stop()
	}
	}

. Just calling Stop() has been sufficient when I've used it.