A runner (Letterbox.Runner) in a process of Letterbox's: each job a
task of Letterbox.TaskSupervisor, tried again with a backoff (one
second, then doubling, at most a minute), up to :max_attempts times.
A job it has when the app stops is not run, but its message keeps its
state in its row: Letterbox.resume/0 gives the messages that were
waiting to the runner again (Letterbox.Sweeper does it on start).