Redmine #6623: Fix agents constantly retrying long running promises.

The expireafter default should be two hours, but it was erronously set
to one minute in the initialization code, so remove that and stick to
the globally set value.

This fixes the problem where, when transferring a large file that
takes longer than the execution interval to finish, no agent will ever
succeed. What happens is that if a second cf-agent starts downloading
the same file, locking will not prevent the attempt, despite already
being in progress by the first agent. No agent will ever succeed,
because the second agent will delete the temporary file created by the
first agent, then create its own temporary file, which will be deleted
by the third agent, and so on.