TODO

TODO: Wait after the refresh API is hit before querying for changes on GitHub.
  It seems like it takes a while (probably ~1min is fine) for mergable pull
  requests to show up. (Or investigate why this is happening.) Note: the API
  must return an HTTP response quickly, even though the main refresh action is
  to be done on a delay.

TODO: Add an option for cleaning away old directories (possibly do this by default?)?
  This could be done on the basis of their age

TODO: Figure out how to re-use partially built CakeML state for new jobs
  Re-use should be possible for the prefix of previously finished directories
  that have not changed

TODO: Make server stop jobs that have been running too long
  This could be done as part of the refresh action.

TODO: Replace calls to the OS shell with SML Basis functionality
  In worker.sml there is a call to OS.Process.system that passes quite a lot of
  work off to the shell: setting environment variables, executing a subprocess,
  and redirecting standard streams. This could all be implemented in the SML
  Basis, in particular using Posix structure, instead. Doing so might be more
  reliable?