Stop job service when pipeline execution finishes

Currently, job servers are shut down when the Python script exits [1]. A better long-term solution would be to shut them down instead when the pipeline is finished executing, such as [2]. This will put resource management in a common code path that is less error-prone.

[1] https://github.com/apache/beam/blob/c5f43342f914fc8ff367b86fb9294c38436ed3ce/sdks/python/apache_beam/runners/portability/job_server.py#L73

[2] https://github.com/apache/beam/blob/c5f43342f914fc8ff367b86fb9294c38436ed3ce/sdks/python/apache_beam/runners/portability/portable_runner.py#L451