Least load balancer (#2999)

* v5: Health Check & LeastLoad Strategy (rebased from 2c5a71490368500a982018a74a6d519c7e121816)

Some changes will be necessary to integrate it into V2Ray

* Update proto

* parse duration conf with time.Parse()

* moving health ping to observatory as a standalone component

* moving health ping to observatory as a standalone component: auto generated file

* add initialization for health ping

* incorporate changes in router implementation

* support principle target output

* add v4 json support for BurstObservatory & fix balancer reference

* update API command

* remove cancelled API

* return zero length value when observer is not found

* remove duplicated targeted dispatch

* adjust test with updated structure

* bug fix for observer

* fix strategy selector

* fix strategy least load

* Fix ticker usage

ticker.Close does not close ticker.C

* feat: Replace default Health Ping URL to HTTPS (#1991)

* fix selectLeastLoad() returns wrong number of nodes (#2083)

* Test: fix leastload strategy unit test

* fix(router): panic caused by concurrent map read and write (#2678)

* Clean up code

---------

Co-authored-by: Jebbs <qjebbs@gmail.com>
Co-authored-by: Shelikhoo <xiaokangwang@outlook.com>
Co-authored-by: 世界 <i@sekai.icu>
Co-authored-by: Bernd Eichelberger <46166740+4-FLOSS-Free-Libre-Open-Source-Software@users.noreply.github.com>
Co-authored-by: 秋のかえで <autmaple@protonmail.com>
Co-authored-by: Rinka <kujourinka@gmail.com>

app/router/balancing_override.go

@@ -0,0 +1,50 @@
+package router
+
+import (
+	sync "sync"
+)
+
+func (r *Router) OverrideBalancer(balancer string, target string) error {
+	var b *Balancer
+	for tag, bl := range r.balancers {
+		if tag == balancer {
+			b = bl
+			break
+		}
+	}
+	if b == nil {
+		return newError("balancer '", balancer, "' not found")
+	}
+	b.override.Put(target)
+	return nil
+}
+
+type overrideSettings struct {
+	target string
+}
+
+type override struct {
+	access   sync.RWMutex
+	settings overrideSettings
+}
+
+// Get gets the override settings
+func (o *override) Get() string {
+	o.access.RLock()
+	defer o.access.RUnlock()
+	return o.settings.target
+}
+
+// Put updates the override settings
+func (o *override) Put(target string) {
+	o.access.Lock()
+	defer o.access.Unlock()
+	o.settings.target = target
+}
+
+// Clear clears the override settings
+func (o *override) Clear() {
+	o.access.Lock()
+	defer o.access.Unlock()
+	o.settings.target = ""
+}