Add worker.SetNearHeapLimitCallback

[zhangyuanwei]

Add a worker.SetNearHeapLimitCallback API to monitor third-party code memory usag. This API providing an opportunity to terminate when memory exceeds a threshold.

eg:

package main

import "fmt"
import "time"
import "github.com/zhangyuanwei/v8worker2"

func main() {

	// set v8 heap limit to 10M
	args := []string{"--max-semi-space-size=10", "--max-old-space-size=10"}
	v8worker2.SetFlags(args)

	go func() {
		worker := v8worker2.New(func(msg []byte) []byte {
			return nil
		})

		worker.SetNearHeapLimitCallback(func(currentHeapLimit uint, initialHeapLimit uint) uint {
			fmt.Printf("near heap limit:[currentHeapLimit:%d, initialHeapLimit:%d]\n", currentHeapLimit, initialHeapLimit)
			worker.TerminateExecution()
			return currentHeapLimit + (10 * 1024 * 1024)
		})

		fmt.Println("worker start")
                // This code will cause a fatal error (OOM)
		worker.Load("oom.js", "var strArr = [\"abcd\"]; while(true){ strArr.push(strArr.join()); }")
		fmt.Println("worker end")
	}()

	for {
		time.Sleep(time.Second)
	}
}