Timeout consistency

Here is an attempt at fixing #63 by introducing timeouts for the BGG cache at two places:

• Total time spent in retrieveBGGThings, including all chunks.
• Total time spent in requestBGG, including all retries.

Other changes are:

• Reworked chunking so that it is possible to make partial progress in refreshing the cache even if there is a timeout.
  • Unfortunately, I had to intertwine request logic and caching logic for this.
  • For now, the combined logic is in the BGG module, but we could consider reworking the cache module as a cache implementation that takes care of timeouts and chunking as well.
• Reworked the delay logic because the timeout function didn't seem to play well with forking a thread that modifies a TMVar after some time. (The forked thread seemed to die without our code being able to catch an exception, so I couldn't find a way to "release the request lock" on timeout.)
• Made retrieval of BGG IDs from all instances a periodic action.
• (Fixed parsing of time configuration.)

While this solution seems to work, it is rather hacky, so I'm open to suggestions for improvement and even completely different approaches. One thing I thought of was specifying the timeout "dynamically" whenever retrieveBGGThings is called in a Snap handler, but I couldn't find a find an easy way of getting the "remaining time" in a Snap handler (and didn't bother to do the time tracking in our code).