Make mmutex actually acquire and release file locks

mmutex/lib/flock.go

-// 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")
-}