replaced GStaticMutex with mlpl::MutexLock in ConfigManager.

GStaticMutex is deprecated for up-to-date GLIB. But GMutex is
not fully compatible with the new (2.36) and the old (2.22).
We have to build asura both on Ubuntu 13.04 and CentOS 6.3.
So we use the mlpl::MutexLock that can be used on both platforms.