Async youtube-dl

[tsani]

Because youtube-dls are done synchronously (in apollo-dl) nginx sometimes kills the connection because it thinks upstream is dead.

[tsani]

This is actually tricky to do because the youtube-dl routine required working directory locking, which we currently have no way to do nicely via the Job monad. It could be interesting to pass an arbitrary constraint as a type parameter to Job so that when the job is run, we require that this constraint be satisfied. This constraint can be a monad class imbuing our job with the ability to acquire the directory lock.