backend: reuse timer in run().
backend: reuse timer in run().

Benchmarks:

```
import (
	"testing"
	"time"
)

func BenchmarkTimeAfter(b *testing.B) {
	b.ReportAllocs()
	for n := 0; n < b.N; n++ {
		select {
		case <- time.After(1 * time.Millisecond):
		}
	}
}

func BenchmarkTimerReset(b *testing.B) {
	b.ReportAllocs()
	t := time.NewTimer(1 * time.Millisecond)
	for n := 0; n < b.N; n++ {
		select {
		case <- t.C:
		}
		t.Reset(1 * time.Millisecond)
	}
}
```

Running reveals that each loop results in 3 allocs:

```
BenchmarkTimeAfter-4 	    2000	   1112134 ns/op	     192 B/op	       3 allocs/op
BenchmarkTimerReset-4	    2000	   1109774 ns/op	       0 B/op	       0 allocs/op
```

armstrongli/etcd,swingbach/etcd,nekto0n/etcd,etcd-io/etcd,hongchaodeng/etcd,linuxcer/etcd,bluebreezecf/etcd,glycerine/etcd,joshix/etcd,rchicoli/etcd,zaphod-concur/etcd,bdarnell/etcd,vimalk78/etcd,coreos/etcd,YuleiXiao/etcd,jonboulle/etcd,mqliang/etcd,visheshnp/etcd,tguangch/etcd,rnaveiras/etcd,endocode/etcd,glevand/coreos--etcd,fanminshi/etcd,endocode/etcd,armstrongli/etcd,qqsun8819/etcd,a-robinson/etcd,akauppi/etcd,a-robinson/etcd,westhood/etcd,heyitsanthony/etcd,wantonsolutions/dinv-etcd,StorReduce/etcd,mqliang/etcd,jlamillan/etcd,ajityagaty/etcd,handsomesun/etcd,visheshnp/etcd,fanminshi/etcd,lumjjb/etcd,dannysauer/etcd,xiang90/etcd,mqliang/etcd,etcd-io/etcd,mkumatag/etcd,jonboulle/etcd,sofuture/etcd,sebrandon1/etcd,rchicoli/etcd,lumjjb/etcd,dannysauer/etcd,li-ang/etcd,glycerine/etcd,CrazyJvm/etcd,ajityagaty/etcd,jlamillan/etcd,wantonsolutions/dinv-etcd,insequent/etcd,yudai/etcd,luxas/etcd,ravigadde/etcd,yudai/etcd,ravigadde/etcd,akauppi/etcd,chipironcin/etcd,linuxcer/etcd,lorneli/etcd,heyitsanthony/etcd,nekto0n/etcd,bdarnell/etcd,gyuho/etcd,joshix/etcd,StorReduce/etcd,chipironcin/etcd,rnaveiras/etcd,purpleidea/etcd,westhood/etcd,mkumatag/etcd,qqsun8819/etcd,mickep76/etcd,CrazyJvm/etcd,mkumatag/etcd,OyTao/etcd_learning,glevand/coreos--etcd,swingbach/etcd,etcd-io/etcd,handsomesun/etcd,hongchaodeng/etcd,ktateish/etcd,xiang90/etcd,plasticbox/etcd,sebrandon1/etcd,sofuture/etcd,insequent/etcd,vimalk78/etcd,purpleidea/etcd,rnaveiras/etcd,wantonsolutions/dinv-etcd,luxas/etcd,li-ang/etcd,zaphod-concur/etcd,tguangch/etcd,ajityagaty/etcd,plasticbox/etcd,lorneli/etcd,OyTao/etcd_learning,whuwxl/etcd,gyuho/etcd,sinsharat/etcd,cockroachdb/etcd,whuwxl/etcd,YuleiXiao/etcd,coreos/etcd,sinsharat/etcd,cockroachdb/etcd,ktateish/etcd,bluebreezecf/etcd,mickep76/etcd