Job_impl: make the getConnection method more flexible

Actually the getConnection() method is available only with the Class Job_Impl. This makes it impossible to use the connection to the JS database in a pre-/post-Processing class.

To use this method in a pre-/post-processing the job_impl has been modified so, that getConnection() always checks, wether a connection is established and when not, try to establish this.

One can use this functionality for pre-/post-processing if the class is an extension of "JobSchedulerAdapterClass" instead of "JobSchedulerJob". This class implements the "monitor_impl" and the "job_impl" in a single class and therefore allows to use getConnection() as well.

Just another issue is, that all jobs which are derived from JobSchedulerJob are creating a connection to the database, even if this is not needed.