Add support for configuring multiple SignalR service instances

This solves a couple of problems:

• Ability to increase the scale by adding more Azure SignalR instances and distribute messages across those instances
• Ability to support multi-region fan out

There are 2 major pieces to this:

• Supporting multiple connection strings
• Selecting a single connection string on negotiate based on the incoming request (user extensible)

The negotiate payload will by default round robin and select a single end point that the client will connect to. Instead of opening N connections to a single service instance, we’ll be opening N connections to M service instances. Each outgoing message should be sent to a single connection on each service instance (see https://github.com/Azure/azure-signalr/blob/25bcc7bd4b3fcd7209578d8138ff053e88490276/src/Microsoft.Azure.SignalR/HubHost/ServiceConnectionManager.cs#L30)

A couple of services need to be tweaked to no longer assume a single connection string (like IServiceEndpointProvider) and we need to change the IServiceConnectionManager implementation to fan out to all servers.

[robynt34]

Is there an update on this feature?

[vicancy]

Official instruction on this is here https://docs.microsoft.com/en-us/azure/azure-signalr/signalr-howto-scale-multi-instances