[#78839768] Fixup jdbc driver mapping for development+java7

 - With java 7 (but not 6) when running in development mode (cache_classes false)
   neither JavaSql::DriverManager#getConnection nor Sequel jdbc driver mapping are able
   to get the correct driver for drivers not included in sequel. In production where
   classes are not reloaded, sequel was able fetch the driver from the now removed
   monkey-patch.
 - Now, structure the additonal key => driver mappings to be used a sequel extension
   and load it when instantiating a JdbcConnection. This allows the drivers to be found
   in development with java7
 - It is still unknown why DriverManager#getConnection works under java 6 but not 7 with
   cache_classes = false