fix /event response stop immediately

[realityone]

Swarm /events API may be will stop immediately at sometime.

After digging these codes I found that we should be carefully to stop a timer. A timer maybe has already expired, even the duration is zero.

We can write an example to prove it:

package main

import (
	"fmt"
	"time"
)

func main() {

	for i := 0; i < 100; i++ {
		go func(c int) {
			timer := time.NewTimer(0)
			if !timer.Stop() {
				fmt.Printf("Timer stop failed at, %d\n", c)
			}
		}(i)
	}
}

Save as timerTest.go and go run with it:

[realityone@rEimu ~/Desktop]$ go run timerTest.go 
Timer stop failed at, 27
[realityone@rEimu ~/Desktop]$ go run timerTest.go 
Timer stop failed at, 41
[realityone@rEimu ~/Desktop]$ go run timerTest.go 
Timer stop failed at, 32
Timer stop failed at, 1
Timer stop failed at, 62
[realityone@rEimu ~/Desktop]$ go run timerTest.go 
Timer stop failed at, 13
Timer stop failed at, 57
Timer stop failed at, 83
[realityone@rEimu ~/Desktop]$ 

So we have to check the timer.Stop return value and drain the channel manually.

Signed-off-by: realityone realityone@me.com

[dongluochen]

LGTM

[dongluochen]

Ping @nishanttotla @allencloud

[nishanttotla]

LGTM

[nishanttotla]

Thanks @realityone!