Also cache the username to avoid copying the connections array

The assets modification processor gets called a lot, and checking
for the user in the callbacks means a copy of the connections array
is made every single time. Add an event to signal that the connections
array has changed instead so we can update the user whenever that happens
and avoid the copy

Author: shana

src/GitHub.Api/Authentication/IKeychain.cs

@@ -1,4 +1,5 @@
-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
 using System.Threading.Tasks;
 
 namespace GitHub.Unity
@@ -17,5 +18,7 @@ public interface IKeychain
         bool HasKeys { get; }
         bool NeedsLoad { get; }
         void SetToken(UriString host, string token);
+
+        event Action ConnectionsChanged;
     }
 }