raft: reuse "last index" in "appendEntry"
raft: reuse "last index" in "appendEntry"

No need to call "lastIndex" again.
"append" call already returns "lastIndex".

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

sinsharat/etcd,bdarnell/etcd,lorneli/etcd,mkumatag/etcd,dannysauer/etcd,coreos/etcd,rnaveiras/etcd,glevand/coreos--etcd,sebrandon1/etcd,mqliang/etcd,cockroachdb/etcd,ajityagaty/etcd,whuwxl/etcd,mkumatag/etcd,sebrandon1/etcd,fanminshi/etcd,rnaveiras/etcd,joshix/etcd,etcd-io/etcd,ajityagaty/etcd,coreos/etcd,vimalk78/etcd,mqliang/etcd,gyuho/etcd,lorneli/etcd,glevand/coreos--etcd,dannysauer/etcd,etcd-io/etcd,mqliang/etcd,bdarnell/etcd,fanminshi/etcd,li-ang/etcd,ajityagaty/etcd,sinsharat/etcd,xiang90/etcd,mkumatag/etcd,rnaveiras/etcd,xiang90/etcd,li-ang/etcd,whuwxl/etcd,joshix/etcd,cockroachdb/etcd,gyuho/etcd,vimalk78/etcd,etcd-io/etcd