Setup a gatherer which collects statistics about background writer

Most changes refer to the feature gathering bgwriter statistics. We want to use sproc wrapper for database access. This required some changes in the dependencies, especially the postgresql jdbc version. The name changes in pom were "forced" by codevalidator. I chose the DriverManagerDataSource in order to get a similar behavior as in all the other gatherers (new connection for each fetch). The sql/schema_upgrade/01_bgwriter.sql is a proposition how to evolve the schema.