Move logging to a script.

I want to display upload status in the dashboard.  The easiest way to do
this is to capture stdout, as that's where rsync progress is displayed.

To do this, we'll need to generate log entries from the wget Lua script
*and* the pipeline.  Easiest way to commonize the behavior: move it to
Redis.

As part of this commit, I've introduced a caching script executor in
Lua.
No, really record when a job finished.

(The os module isn't available from Redis' Lua environment.)
Make finished_at an integral UNIX timestamp.

(Actually, that's a lie.  We're going to get strings, but they'll be
strings that can be coerced to integers.)