rafthttp: use 'transport.IsClosedConnError'
rafthttp: use 'transport.IsClosedConnError'

Signed-off-by: Gyu-Ho Lee <3454051ee3341eb69bdf6fca40899cdad4d86496@gmail.com>

vimalk78/etcd,ajityagaty/etcd,mickep76/etcd,rnaveiras/etcd,joshix/etcd,glevand/coreos--etcd,sebrandon1/etcd,etcd-io/etcd,heyitsanthony/etcd,mkumatag/etcd,visheshnp/etcd,mqliang/etcd,glevand/coreos--etcd,plasticbox/etcd,lorneli/etcd,armstrongli/etcd,bdarnell/etcd,fanminshi/etcd,sinsharat/etcd,mkumatag/etcd,ajityagaty/etcd,coreos/etcd,YuleiXiao/etcd,sebrandon1/etcd,yudai/etcd,gyuho/etcd,bdarnell/etcd,rnaveiras/etcd,mkumatag/etcd,plasticbox/etcd,sinsharat/etcd,li-ang/etcd,bluebreezecf/etcd,li-ang/etcd,a-robinson/etcd,etcd-io/etcd,etcd-io/etcd,armstrongli/etcd,rnaveiras/etcd,vimalk78/etcd,mqliang/etcd,whuwxl/etcd,lorneli/etcd,whuwxl/etcd,heyitsanthony/etcd,bluebreezecf/etcd,fanminshi/etcd,visheshnp/etcd,xiang90/etcd,xiang90/etcd,joshix/etcd,ajityagaty/etcd,dannysauer/etcd,gyuho/etcd,a-robinson/etcd,cockroachdb/etcd,cockroachdb/etcd,mqliang/etcd,YuleiXiao/etcd,yudai/etcd,mickep76/etcd,akauppi/etcd,akauppi/etcd,coreos/etcd,dannysauer/etcd