Differentiation between destroy and disconnect

Disconnect should only disconnect the pool (non-destructive close of the connection).  Destroy should truncate the tables you added data to (destructive close of the connection).  This will shorten the amount of code needed to clean up after each series of tests but is opt-in by using the `destroy` method.

This is a last-resort, as my attempt at using transactions was unfruitful.  You would have to use the same knex connection provided by sql-fixtures to run your tests to make it possibly-maybe even work a little.
if truncate ends up being too database specific, maybe a PR to knex with truncate support would be the way to go. Or I also don't mind just truncating outside of sql-fixtures, which is what I'm currently doing.