Recently it has happened two times when people tried to deploy the new cluster but postgres data directory wasn't empty and also wasn't valid. In this case Patroni was still creating initialize key in DCS and trying to start the postgres up.
Now it will complain about non-empty invalid postgres data directory and exit.
Close https://github.com/zalando/patroni/issues/1216
the main purpose of this PR is simplifying #1068
It is mostly necessary for future support of pg12, where there will be no recovery.conf anymore, but `keep_existing_recovery_conf` parameter still needs to be supported due to backward compatibility.
* Convert postgresql.py into a package
* Factor out cancellable process into a separate class
* Factor out connection handler into a separate class
* Move postmaster into postgresql package
* Factor out pg_rewind into a separate class
* Factor out bootstrap into a separate class
* Factor out slots handler into a separate class
* Factor out postgresql config handler into a separate class
* Move callback_executor into postgresql package
This is just a careful refactoring, without code changes.
