fix(server): close old log file after swap in SetOutputFile

Swap outputFile before closing the previous file handle, so no
read of outputFile can observe a dangling pointer to a closed
file.  Also wrap the file-open in the mutex (already done) for
atomicity of the entire file rotation sequence.

log/log.go

@@ -173,10 +173,11 @@ func SetOutputFile(path string) error {
 		return err
 	}
 	SetOutput(f)
-	if outputFile != nil {
-		_ = outputFile.Close()
-	}
+	prev := outputFile
 	outputFile = f
+	if prev != nil {
+		_ = prev.Close()
+	}
 	return nil
 }