raft: use mutex in "SetLogger" to avoid race conditions in tests

Signed-off-by: Gyuho Lee <leegyuho@amazon.com>

raft/logger.go

@@ -19,6 +19,7 @@ import (
 	"io/ioutil"
 	"log"
 	"os"
+	"sync"
 )
 
 type Logger interface {
@@ -41,11 +42,16 @@ type Logger interface {
 	Panicf(format string, v ...interface{})
 }
 
-func SetLogger(l Logger) { raftLogger = l }
+func SetLogger(l Logger) {
+	raftLoggerMu.Lock()
+	raftLogger = l
+	raftLoggerMu.Unlock()
+}
 
 var (
 	defaultLogger = &DefaultLogger{Logger: log.New(os.Stderr, "raft", log.LstdFlags)}
 	discardLogger = &DefaultLogger{Logger: log.New(ioutil.Discard, "", 0)}
+	raftLoggerMu  sync.Mutex
 	raftLogger    = Logger(defaultLogger)
 )