pull: Rework threading communication model

Previously, I've observed bugs where we either:
1) Exit too early, leaving undownloaded objects
2) Hang while downloading

This rewrite hopefully fixes both.

src/libotutil/otutil.h

@@ -37,7 +37,7 @@
     }                                                      \
   } G_STMT_END;
 
-#include <ot-worker-queue.h>
+#include <ot-waitable-queue.h>
 #include <ot-local-alloc.h>
 #include <ot-keyfile-utils.h>
 #include <ot-gio-utils.h>