Some MySQL improvements

Here are some changes to the MySQL backend that should help with #540 and similar:

  * remove a proper 503 when there's a MySQL connection error
  * add ability to limit connection queue backlog
  * re-create the pool if it happens to be torn down due to excessive errors

I'm open to discussion on that last point.  I can see us getting loop where we're constantly re-creating a pool, saturating it, removing it and re-creating it.  But I'm not sure the alternatives are any better.  At least this way to app should recover automatically once the db is repaired or the load decreases or whatever.

@chilts r?