pull: Asynchronous metadata fetch

Create a worker thread for processing metadata, reserving the main
thread for HTTP requests.

This can create a very significant efficiency win for large pull
requests since we are much more likely to keep a full pipeline open.

The status display is also nicer now.

src/libotutil/otutil.h

@@ -35,6 +35,7 @@
     }                                                      \
   } G_STMT_END;
 
+#include <ot-worker-queue.h>
 #include <ot-local-alloc.h>
 #include <ot-keyfile-utils.h>
 #include <ot-gio-utils.h>