log-backup: add mutual execution for log truncating.

[YuJuncen]

Enhancement

Sometimes, if we are running parallel truncating tasks, we may encounter some strange things: say, a meta file being truncated in one truncate task, but yet another one also wants to delete it. The latter one may finish with a failed deletion and will greatly confuse our users.

[YuJuncen]

A simple lock like backup.lock might be good enough. Except for some rare race conditions.

[YuJuncen]

For a more general case, we can make interfaces like:

type Locker interface {
  Lock(ctx context.Context, path string, hint string) (UnlockFunc, error)
}

type UnlockFunc func(ctx context.Context) error

And implement the basic backup.lock version for general external storage.
Then we can implement platform-specified version for each storage implementation. Say, ObjectLock for S3, flock for POSIX FS.