client: add response unmarshal benchmark
client: add response unmarshal benchmark

The benchmark result:

```
BenchmarkSmallResponseUnmarshal	  10000	   164524 ns/op
BenchmarkManySmallResponseUnmarshal	    100	 13916636 ns/op
BenchmarkMediumResponseUnmarshal	   1000	  1974295 ns/op
BenchmarkLargeResponseUnmarshal	     20	 80462001 ns/op
ok		github.com/coreos/etcd/client	7.777s
```

tguangch/etcd,wantonsolutions/dinv-etcd,asim/etcd,endocode/etcd,chipironcin/etcd,wowsoso/etcd,rnaveiras/etcd,delkyd/etcd,purpleidea/etcd,swingbach/etcd,fgbreel/etcd,sinsharat/etcd,wantonsolutions/dinv-etcd,hroyrh/etcd,bluebreezecf/etcd,jchauncey/etcd,glevand/coreos--etcd,tamird/etcd,chenbaihu/etcd,sayden/etcd,bdarnell/etcd,luxas/etcd,honeyqa/etcd,yichengq/etcd,hongchaodeng/etcd,plasticbox/etcd,Timer/etcd,linuxcer/etcd,giantswarm/etcd,qqsun8819/etcd,oscarrr110/etcd,msanterre/etcd,onlyjob/etcd,cockroachdb/etcd,cchamplin/etcd,kkaneda/etcd,luxas/etcd,lumjjb/etcd,siddontang/etcd,jeremyeder/etcd,chenbaihu/etcd,li-ang/etcd,ravigadde/etcd,mkumatag/etcd,YuleiXiao/etcd,lorneli/etcd,fasaxc/etcd,hongbinz/etcd,StorReduce/etcd,Winslett/etcd,OyTao/etcd_learning,ajityagaty/etcd,westhood/etcd,mqliang/etcd,nekto0n/etcd,kkaneda/etcd,mqliang/etcd,akauppi/etcd,Winslett/etcd,etcd-io/etcd,beni55/etcd,mkumatag/etcd,jlamillan/etcd,xychu/etcd,jonboulle/etcd,Eagle-X/etcd,jlamillan/etcd,heyitsanthony/etcd,westhood/etcd,hongbinz/etcd,arthurcolle/etcd,armstrongli/etcd,CrazyJvm/etcd,combk8s/etcd,HideoYamauchi/etcd,sebrandon1/etcd,mike-lerch/etcd,YuleiXiao/etcd,endocode/etcd,fanminshi/etcd,sofuture/etcd,achanda/etcd,siddontang/etcd,mike-lerch/etcd,rchicoli/etcd,onlyjob/etcd,dannysauer/etcd,glevand/coreos--etcd,OyTao/etcd_learning,whuwxl/etcd,joshix/etcd,plasticbox/etcd,mkumatag/etcd,arthurcolle/etcd,xdlinear/etcd,betawaffle/etcd,joshix/etcd,StorReduce/etcd,vimalk78/etcd,xdlinear/etcd,giantswarm/etcd,Yhgenomics/etcd,fanminshi/etcd,iamqizhao/etcd,mickep76/etcd,coreos/etcd,beni55/etcd,rnaveiras/etcd,combk8s/etcd,mickep76/etcd,etcd-io/etcd,oscarrr110/etcd,betawaffle/etcd,Yhgenomics/etcd,raoofm/etcd,CrazyJvm/etcd,zaphod-concur/etcd,rchicoli/etcd,jchauncey/etcd,bdarnell/etcd,xiang90/etcd,swingbach/etcd,wowsoso/etcd,improbable-io/etcd,TheHippo/etcd,qqsun8819/etcd,sebrandon1/etcd,whuwxl/etcd,hroyrh/etcd,ajityagaty/etcd,handsomesun/etcd,shawnps/etcd,heyitsanthony/etcd,ktateish/etcd,pauldallen/etcd,fasaxc/etcd,gityf/etcd,yudai/etcd,a-robinson/etcd,sayden/etcd,jefby/etcd,dannysauer/etcd,chipironcin/etcd,Timer/etcd,tguangch/etcd,gae123/etcd,vimalk78/etcd,fasaxc/etcd,handsomesun/etcd,svdata/etcd,gouyang/etcd,gyuho/etcd,li-ang/etcd,Eagle-X/etcd,cockroachdb/etcd,wantonsolutions/dinv-etcd,insequent/etcd,armstrongli/etcd,tamird/etcd,akauppi/etcd,jonboulle/etcd,dnaeon/etcd,achanda/etcd,hongchaodeng/etcd,pauldallen/etcd,visheshnp/etcd,insequent/etcd,sofuture/etcd,delkyd/etcd,raoofm/etcd,improbable-io/etcd,asim/etcd,cchamplin/etcd,fgbreel/etcd,ravigadde/etcd,svdata/etcd,lumjjb/etcd,etcd-io/etcd,glycerine/etcd,xiang90/etcd,ajityagaty/etcd,msanterre/etcd,a-robinson/etcd,honeyqa/etcd,sinsharat/etcd,buaazp/etcd,yudai/etcd,ktateish/etcd,gyuho/etcd,rnaveiras/etcd,shawnps/etcd,gouyang/etcd,bluebreezecf/etcd,visheshnp/etcd,linuxcer/etcd,jefby/etcd,TheHippo/etcd,nekto0n/etcd,purpleidea/etcd,gityf/etcd,yichengq/etcd,jeremyeder/etcd,glycerine/etcd,iamqizhao/etcd,HideoYamauchi/etcd,coreos/etcd,dnaeon/etcd,mqliang/etcd,gae123/etcd,lorneli/etcd,zaphod-concur/etcd,xychu/etcd,buaazp/etcd