Refactor transports to help enable graceful shutdown (#142)

- This also paves the way for better multi-session support
- We should definitely rethink names for the transport API
- For now, I kept the names similar as possible, so we can
  focus on the API shape

Co-authored-by: Stephen Toub <stoub@microsoft.com>

src/ModelContextProtocol/Protocol/Transport/TransportBase.cs

@@ -26,7 +26,7 @@ public abstract class TransportBase : ITransport
             SingleReader = true,
             SingleWriter = true,
         });
-        _logger = (ILogger?)loggerFactory?.CreateLogger<TransportBase>() ?? NullLogger.Instance;
+        _logger = loggerFactory?.CreateLogger(GetType()) ?? NullLogger.Instance;
     }
 
     /// <inheritdoc/>