Feat: aria2 download and transfer in slave node (#1040)

* Feat: retrieve nodes from data table

* Feat: master node ping slave node in REST API

* Feat: master send scheduled ping request

* Feat: inactive nodes recover loop

* Modify: remove database operations from aria2 RPC caller implementation

* Feat: init aria2 client in master node

* Feat: Round Robin load balancer

* Feat: create and monitor aria2 task in master node

* Feat: salve receive and handle heartbeat

* Fix: Node ID will be 0 in download record generated in older version

* Feat: sign request headers with all `X-` prefix

* Feat: API call to slave node will carry meta data in headers

* Feat: call slave aria2 rpc method from master

* Feat: get slave aria2 task status
Feat: encode slave response data using gob

* Feat: aria2 callback to master node / cancel or select task to slave node

* Fix: use dummy aria2 client when caller initialize failed in master node

* Feat: slave aria2 status event callback / salve RPC auth

* Feat: prototype for slave driven filesystem

* Feat: retry for init aria2 client in master node

* Feat: init request client with global options

* Feat: slave receive async task from master

* Fix: competition write in request header

* Refactor: dependency initialize order

* Feat: generic message queue implementation

* Feat: message queue implementation

* Feat: master waiting slave transfer result

* Feat: slave transfer file in stateless policy

* Feat: slave transfer file in slave policy

* Feat: slave transfer file in local policy

* Feat: slave transfer file in OneDrive policy

* Fix: failed to initialize update checker http client

* Feat: list slave nodes for dashboard

* Feat: test aria2 rpc connection in slave

* Feat: add and save node

* Feat: add and delete node in node pool

* Fix: temp file cannot be removed when aria2 task fails

* Fix: delete node in admin panel

* Feat: edit node and get node info

* Modify: delete unused settings

pkg/balancer/roundrobin.go

@@ -0,0 +1,30 @@
+package balancer
+
+import (
+	"reflect"
+	"sync/atomic"
+)
+
+type RoundRobin struct {
+	current uint64
+}
+
+// NextPeer 返回轮盘的下一节点
+func (r *RoundRobin) NextPeer(nodes interface{}) (error, interface{}) {
+	v := reflect.ValueOf(nodes)
+	if v.Kind() != reflect.Slice {
+		return ErrInputNotSlice, nil
+	}
+
+	if v.Len() == 0 {
+		return ErrNoAvaliableNode, nil
+	}
+
+	next := r.NextIndex(v.Len())
+	return nil, v.Index(next).Interface()
+}
+
+// NextIndex 返回下一个节点下标
+func (r *RoundRobin) NextIndex(total int) int {
+	return int(atomic.AddUint64(&r.current, uint64(1)) % uint64(total))
+}