Dealing with unreliable storage

[Tratcher]

One of the larger complaints with Asp.Net 4 Session is that when the session provider goes down it takes down the entire site, when session may not be a critical component of the site.

We have not hardened the new implementation to handle this scenario. There are several TODOs in the code (https://github.com/aspnet/Session/blob/dev/src/Microsoft.AspNetCore.Session/DistributedSession.cs#L105) around suppressing session load failures. We even had an ISession.IsAvailable property to indicate this, but I think it was removed when nobody used it.