diego staging completion handler is idempotent [#80259174]

Signed-off-by: Johannes Petzold <jpetzold@pivotal.io>
Refactor Backends into three

* Stagers - know about staging
* Runners - know about running
* InstancesReporters - know how to report

[#80165482]
Merge remote-tracking branch 'origin/master' into diego-dea-alternating-80165482

Conflicts:
	lib/cloud_controller/diego/docker/staging_completion_handler.rb
	lib/cloud_controller/diego/traditional/staging_completion_handler.rb