etcdserver: allow zero kv index for cluster upgrade
etcdserver: allow zero kv index for cluster upgrade

If a user upgrades etcd from 2.3.x to 3.0 and shutdown the
cluster immediately without triggering any new backend writes,
then the consistent index in backend would be zero.

The user cannot restart etcdserver due to today's strick index
match checking. We now have to lose this a bit for this case.

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