[PDI-11946] /kettle/runJob is limited by the HTTP session timeout

If a job run via /kettle/runJob exceeds HTTP timeout, repository connection may be invalidated.
This fix creates a dedicated repo connection for each job using SlaveServerConfig.

Unit test provided

http://jira.pentaho.com/browse/PDI-11926
Forward-port of http://jira.pentaho.com/browse/SP-1266