Make mmutex actually acquire and release file locks

mmutex/lib/flock.go

Index: mmutex/lib/flock.go
diff --git a/mmutex/lib/flock.go b/mmutex/lib/flock.go
deleted file mode 100644
index 24c6c2f8045e1834da37eb9f5c9ce056c7bcc66f..0000000000000000000000000000000000000000
--- a/mmutex/lib/flock.go
+++ /dev/null
@@ -1,21 +0,0 @@
-// Copyright 2017 The LUCI Authors. All rights reserved.
-// Use of this source code is governed under the Apache License, Version 2.0
-// that can be found in the LICENSE file.
-
-package lib
-
-import (
-	"os"
-
-	"github.com/luci/luci-go/common/errors"
-)
-
-// TODO(charliea): Add timeout parameter to this function.
-func AcquireExclusiveLock(path string) error {
-	_, err := os.OpenFile(path, os.O_APPEND, 0666)
-	if os.IsNotExist(err) {
-		return errors.Reason("lock file does not exist", path).Err()
-	}
-
-	panic("implement lock acquisition")
-}