Allow null values to be cached and refine TryGet method (see #155)

[EnricoMassone]

I have introduced the following changes:

• now the IAppCache.Add method allows to cache nullvalues. No exception is thrown.
• changed the signature of IAppCache.TryGetValue from bool TryGetValue<T>(string key, out object value); to bool TryGetValue<T>(string key, out T value);
• modified the implementation of IAppCache.TryGetValue so that values cached by using IAppCache.GetOrAdd and / or IAppCache.GetOrAddAsync are handled correctly (the actual cached value is returned insted of the lazy wrapper).
• augmented the IAppCache interface with the method bool TryGetValueAsync<T>(string key, out Task<T> value); used to try fetch values cached by using IAppCache.GetOrAddAsync

See issue #155 for the details.

[EnricoMassone]

Using the IAppCache.TryGetValueAsync method is quite convoluted.

Consider this example:

var cache = new CachingService();

const string key = "my-key";
await cache.GetOrAddAsync(key, async () => 
{
  await Task.Delay(100);
  return "cached value";
}

_ = cache.TryGetValueAsync(key, out var taskForFetchedValue);
var fetchedValue = await taskForFetchedValue; // fetched value is the string "cached value"

The API used must await the Task<T> obtained via the outparameter. Unfortunately we can't await the task for him, by providing an async implementation of IAppCache.TryGetValueAsync, because out parameters are not allowed in async methods.

I'm not a big fan of output parameters, because they are basically parameters used to return values outside of a method. Some people consider them as a bad practice. We can maybe consider to refactor the signature of both IAppCache.TryGetValue and IAppCache.TryGetValueAsync, so that tuples are used instead of out parameters:

 (bool wasKeyFound, T value) TryGetValue<T>(string key);
 Task<(bool wasKeyFound, T value)> TryGetValueAsync<T>(string key);

By doing so, we can write an async implementation of TryGetValueAsync and await the task on behalf of the API user.

[EnricoMassone]

Hi, any news on this ? Did you take a canche to review ?