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/diff_test.go

@@ -60,7 +60,7 @@ func mustTemp(pre, body string) string {
 func ltoa(l *raftLog) string {
 	s := fmt.Sprintf("committed: %d\n", l.committed)
 	s += fmt.Sprintf("applied:  %d\n", l.applied)
-	for i, e := range l.ents {
+	for i, e := range l.allEntries() {
 		s += fmt.Sprintf("#%d: %+v\n", i, e)
 	}
 	return s