Ping the connected host regularly and check other hosts at lower freq.

* This lets us detect when a remote server has been disconnected and
  needs to be restarted, as well as alerting the user if this happens in
  the middle of a replay session.
* Pinging other hosts means the context switcher is reasonably up to
  date if one of them comes up.

renderdocui/Interop/ReplayRenderer.cs

@@ -892,6 +892,9 @@ namespace renderdoc
         [DllImport("renderdoc.dll", CharSet = CharSet.Unicode, CallingConvention = CallingConvention.Cdecl)]
         private static extern void RemoteServer_ShutdownServerAndConnection(IntPtr real);
 
+        [DllImport("renderdoc.dll", CharSet = CharSet.Unicode, CallingConvention = CallingConvention.Cdecl)]
+        private static extern bool RemoteServer_Ping(IntPtr real);
+
         [DllImport("renderdoc.dll", CharSet = CharSet.Unicode, CallingConvention = CallingConvention.Cdecl)]
         private static extern bool RemoteServer_LocalProxies(IntPtr real, IntPtr outlist);
         [DllImport("renderdoc.dll", CharSet = CharSet.Unicode, CallingConvention = CallingConvention.Cdecl)]
@@ -986,6 +989,11 @@ namespace renderdoc
             return ret;
         }
 
+        public bool Ping()
+        {
+            return RemoteServer_Ping(m_Real);
+        }
+
         public string[] LocalProxies()
         {
             IntPtr mem = CustomMarshal.Alloc(typeof(templated_array));