Comment 8 for bug 280578

This bug is a little confused (possible implementation details mixed in with problem statements).

AIUI the issues are twofold:
 - high latency on scanning
 - oopses when a pull succeeds but the branch can't be scanned correctly

For the former, amqp can help a lot (and with parallelism and just-in-time-scaling too). For the latter, I don't see how the situation would change - if pulling succeeds but scanning fails, we need to issue an error somehow. We could just write that to the branch status field today.