.NET Standard compat: Avoid Silo being MarshalByRefObject but support running tests

[jdom]

In our .NET Standard solution I temporarily removed the MarshalByRefObject inheritance from the Silo class, as it's not supported in .NET Standard. See 0a80ae6 for the commit where this was excluded (ignore the test hooks for now, as this work is being done independently and is already in progress by @galvesribeiro).

Nevertheless, the testing infrastructure in the OrleansTestinhHost project (that contains TestCluster) relies on it. This project is still targeting .NET 4.6 in the vNext solution, and is able to use AppDomains, nevertheless it should not rely on the Silo class itself to be the glue between the AppDomains, and instead rely in something like AppDomainSiloHost (that can live in this test project) to inherit from MarshalByRefObject and use the Silo.

In order to open the solution that compiles using .NET Standard (and hence easy to check what is available in that environment), check out this readme

[jdom]

I'm grabbing this one. Will have a PR out tomorrow, but wanted to ping this issue to avoid duplicate effort.

[jdom]

Implemented in #2241