Go: introducing SHM_ACK observer.

Each request processed in a separate goroutine.  In case of OOSM state,
during response write, request goroutine blocks on channel which waits
event from main thread about SHM_ACK message from router.

go/observable.go

@@ -0,0 +1,32 @@
+/*
+ * Copyright (C) NGINX, Inc.
+ */
+
+package unit
+
+import (
+	"sync"
+)
+
+type observable struct {
+	sync.Mutex
+	observers []chan int
+}
+
+func (o *observable) attach(c chan int) {
+	o.Lock()
+	defer o.Unlock()
+
+	o.observers = append(o.observers, c)
+}
+
+func (o *observable) notify(e int) {
+	o.Lock()
+	defer o.Unlock()
+
+	for _, v := range o.observers {
+		v <- e
+	}
+
+	o.observers = nil
+}