Async instead of sync methods in realtime classes

[AndrewVoisovych]

Changes

I changed all synchronous calls to asynchronous calls in interfaces/classes:

• IOnlineClientStore
• IOnlineClientManager / OnlineClientManager
• OnlineClientManagerExtensions
• InMemoryOnlineClientStore
• RedisOnlineClientStore.cs
  And in all places where they are called (including tests).

Removed:

• SignalRRealTimeNotifier.SendNotifications() method and depends:

I also changed the logic of IOnlineClientManager.GetAllByUserId,
namely optimized this by storing a separate hash set of users with references to the connection hash set in Redis.
Added GetAllByUserId / GetAllByUserIdAsync to IOnlineClientStore and RedisOnlineClientStore.

Reasoning

When using the latest version of a project, I noticed that the number of threads in the thread pool grows about x3 times the usual value since synchronous calls create thread pool starvation issues.

First, you can see the graphs from Application Insights, which clearly show the increase in ThreadPool:

The following screenshots are an accurate description of what blocks the application the most, and it turned out to be synchronous methods:

This all means that synchronous calls block and should be replaced with asynchronous calls

About Redis
In the current implementation, all online clients are retrieved from the Redis cache, which is very expensive. For instance in production env, this means transferring ~3 MB of data.

After local changes and testing, the situation has improved, so I would like to see these changes in the framework itself.

[AndrewVoisovych]

@dotnet-policy-service agree

[ismcagdas]

@AndrewVoisovych thank you very much for your contribution.