Loadable cache still allows double calls to a loading function

[advoretsky]

After the work done under

• Avoid parallel running a load function for the same key #242 and
• Load function can be called multiple times for a single cache key before expiration #251
  the code of the loadable cache still doesn't completely prevent double calls to the loading function.

To make it 100% correct the whole logic of the Get function should be put behind a lock. One way to do this is to move everything under the singleFlight call