added round_robin, least_conn and ip_hash load balance support, small refactoring

internal/net/http/loadbalancer/round_robin.go

@@ -0,0 +1,22 @@
+package loadbalancer
+
+import (
+	"net/http"
+	"sync/atomic"
+)
+
+type roundRobin struct {
+	index atomic.Uint32
+}
+
+func (*LoadBalancer) newRoundRobin() impl         { return &roundRobin{} }
+func (lb *roundRobin) OnAddServer(srv *Server)    {}
+func (lb *roundRobin) OnRemoveServer(srv *Server) {}
+
+func (lb *roundRobin) ServeHTTP(srvs servers, rw http.ResponseWriter, r *http.Request) {
+	index := lb.index.Add(1)
+	srvs[index%uint32(len(srvs))].handler.ServeHTTP(rw, r)
+	if lb.index.Load() >= 2*uint32(len(srvs)) {
+		lb.index.Store(0)
+	}
+}