only fetch onee jbo at a time

ofborg/src/bin/builder.rs

@@ -49,6 +49,7 @@ fn main() {
     let cloner = checkout::cached_cloner(Path::new(&cfg.checkout.root));
     let nix = cfg.nix();
 
+    channel.basic_prefetch(1).unwrap();
     channel.basic_consume(
         worker::new(tasks::build::BuildWorker::new(cloner, nix, cfg.nix.system.clone())),
         format!("build-inputs-{}", cfg.nix.system.clone()).as_ref(),

ofborg/src/bin/github-comment-filter.rs

@@ -38,6 +38,7 @@ fn main() {
 
     let mut channel = session.open_channel(2).unwrap();
 
+    channel.basic_prefetch(1).unwrap();
     channel.basic_consume(
         worker::new(tasks::githubcommentfilter::GitHubCommentWorker::new(
             cfg.acl(),

ofborg/src/bin/mass-rebuilder.rs

@@ -69,7 +69,7 @@ fn main() {
     }));
      */
 
-    channel.basic_prefetch(1);
+    channel.basic_prefetch(1).unwrap();
     channel.basic_consume(
         worker::new(mrw),
         "mass-rebuild-check-jobs",