To have better understanding, and make it easier to filter, add the
current pipline class to every message logged in
BulkImports::Pipeline::Runner.

Also:
- add a info log message when the pipeline finishes.
- separete the pipeline step type from its class

** Before
```
{
  "severity":"",
  "time":"",
  "correlation_id":"",
  "bulk_import_entity_id":1,
  "bulk_import_entity_type":"",
  "extractor":""
}
```

** After
```
{
  "severity":"",
  "time":"",
  "correlation_id":"",
  "bulk_import_entity_id":1,
  "bulk_import_entity_type":"",
  "pipeline_class": "",
  "pipeline_step": "",
  "step_class": ""
}
```