Implemented inter-thread SendMessageCallback.

diff --git a/server/trace.c b/server/trace.c
index d9f6833..22b542a 100644
--- a/server/trace.c
+++ b/server/trace.c
@@ -1844,6 +1844,7 @@
     fprintf( stderr, " time=%08x,", req->time );
     fprintf( stderr, " info=%08x,", req->info );
     fprintf( stderr, " timeout=%d,", req->timeout );
+    fprintf( stderr, " callback=%p,", req->callback );
     fprintf( stderr, " data=" );
     dump_varargs_bytes( cur_size );
 }