etcdserver: remove possibly compacted entry look-up

Fix https://github.com/coreos/etcd/issues/7470.

This patch removes unnecessary term look-up in
'createMergedSnapshotMessage', which can trigger panic
if raft entry at etcdProgress.appliedi got compacted
by subsequent 'MsgSnap' messages--if a follower is
being (in this case, network latency spikes), it
could receive subsequent 'MsgSnap' requests from leader.

etcd server-side 'applyAll' routine and raft's Ready
processing routine becomes asynchronous after raft
entries are persisted. And given that raft Ready routine
takes less time to finish, it is possible that second
'MsgSnap' is being handled, while the slow 'applyAll'
is still processing the first(old) 'MsgSnap'. Then raft
Ready routine can compact the log entries at future
index to 'applyAll'. That is how 'createMergedSnapshotMessage'
tried to look up raft term with outdated etcdProgress.appliedi.

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

etcdserver/server_test.go

@@ -615,7 +615,7 @@ func TestApplyMultiConfChangeShouldStop(t *testing.T) {
 		ents = append(ents, ent)
 	}
 
-	_, shouldStop := srv.apply(ents, &raftpb.ConfState{})
+	_, _, shouldStop := srv.apply(ents, &raftpb.ConfState{})
 	if !shouldStop {
 		t.Errorf("shouldStop = %t, want %t", shouldStop, true)
 	}