raft: introduce log storage interface.

This change splits the raftLog.entries array into an in-memory
"unstable" list and a pluggable interface for retrieving entries that
have been persisted to disk. An in-memory implementation of this
interface is provided which behaves the same as the old version;
in a future commit etcdserver could replace the MemoryStorage with
one backed by the WAL.

raft/node_bench_test.go

@@ -27,7 +27,7 @@ func BenchmarkOneNode(b *testing.B) {
 	defer cancel()
 
 	n := newNode()
-	r := newRaft(1, []uint64{1}, 10, 1)
+	r := newRaft(1, []uint64{1}, 10, 1, nil)
 	go n.run(r)
 
 	defer n.Stop()