fix(communication): evict idle SiteStreamGrpcClientFactory (site, endpoint) clients (ScadaBridge#67)

Each cached client records when it was last handed out; a periodic sweep (half of the new
CommunicationOptions.GrpcClientIdleTtl, default 30 min, 0 disables) disposes and removes entries
unused for the TTL that have no live subscription (SiteStreamGrpcClient.ActiveSubscriptionCount).
Lookup and the sweep's check-and-remove share one lock so a handed-out client is never disposed
under its caller; an evicted client is re-created on next GetOrCreate.

src/ZB.MOM.WW.ScadaBridge.Communication/Grpc/SiteStreamGrpcClient.cs

@@ -131,6 +131,12 @@ public class SiteStreamGrpcClient : IAsyncDisposable, IDisposable
         ConnectedHeaderTimeout = options.ConnectedHeaderTimeout;
     }
 
+    /// <summary>
+    /// Number of live subscriptions currently registered on this client. The owning
+    /// <see cref="SiteStreamGrpcClientFactory"/> never evicts a client while this is non-zero.
+    /// </summary>
+    public virtual int ActiveSubscriptionCount => _subscriptions.Count;
+
     /// <summary>
     /// Registers a CancellationTokenSource for a correlation ID. Test-only.
     /// </summary>