FileCache should provide a AddOrGetExisting method which takes a factory

[palenshus]

Currently the AddOrGetExisting method takes a value, which requires you to pre-compute the value whether or not the entry is already in the cache. Instead it should take a generator method (Func<T>) which will generate the value in the instance of a cache miss. See https://github.com/alastairtree/LazyCache for an example of this, copied here:

// Declare (but don't execute) a func/delegate whose result we want to cache
Func<ComplexObjects> complexObjectFactory = () => methodThatTakesTimeOrResources();

// Get our ComplexObjects from the cache, or build them in the factory func 
// and cache the results for next time under the given key
ComplexObjects cachedResults = cache.GetOrAdd("uniqueKey", complexObjectFactory);

It should also provide an async overload for this operation as well