rake db:* fixes

I want a single rake command to run tests: `bundle exec rake db:setup spec`

Before this patch, I observed that `rake db:setup` is leaking database connections that the spec database cleaner was complaining about.

This patch has every rake database operation explicitly disconnect from the database and clear the Sequel connection pool when complete.

Also, it includes a fix to preserve a DATABASE_URL username and/or password when connecting to the 'postgres' database to drop databases.