Support async correctly

[aaubry]

Work to be completed:

• [x] Add CancellationToken to InvalidateAsync
• [x] Add unit tests for RedisCache
• [x] Add unit tests for RedisInvalidatorReceiver
• [x] Add unit tests for RedisInvalidatorSender
• [x] Review MemoryCacheTests (see if more things should be tested)
• [x] Fix compiler warnings
• [x] Update the documentation
• [x] Use Task<T> instead of ValueTask<T> everywhere
• [x] Check for ConfigureAwait(false)
• [x] Add ProtobufSerializer unit tests

• [x] Add JsonStringSerializer unit tests

Note: Please ensure that README.md is up-to-date with any changes made above

[sklivvz]

I've added a note :-D

[aaubry]

I'll start with the ConfigureAwait thing, as I have more context about it.

[aaubry]

I'll add tests to RedisCache.

[aaubry]

I'll add tests for RedisInvalidatorReceiver / RedisInvalidatorSender

[aaubry]

Reviewing MemoryCacheTests