Ensure Dispose is call on every MD5CryptoServiceProvider class usage

Every Get or Set call invokes KetamaNodeLocator.Locate(string key). The memcached keys are hashed through MD5 algorithm to find node they are supposed to be stored on. The current implementation creates a new MD5CryptoServiceProvider class which has underlying SafeHashHandle that registers into finalizer queue. Because the code does not call Dispose on the MD5CryptoServiceProvider instance, the SafeHashHandle remains in finalizer queue till garbage collector disposes it properly.