mvcc: create TxnWrites from TxnRead with NewReadOnlyTxnWrite
mvcc: create TxnWrites from TxnRead with NewReadOnlyTxnWrite

Already used internally by mvcc, but needed by etcdserver txns.

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