ctlv2: save v3 db with v2 data using --with-v3
ctlv2: save v3 db with v2 data using --with-v3

Also strips out v3 data if not given --with-v3.

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