Old versions of `kazoo` immediately discarded all requests to Zookeeper if the connection is in the `SUSPENDED` state. This is absolutely fine because Patroni is handling retries on its own.
Starting from 2.7, kazoo started queueing requests instead of discarding and as a result, the Patroni HA loop was getting stuck until the connection to Zookeeper is reestablished, causing no demote of the Postgres.
In order to return to the old behavior we override the `KazooClient._call()` method.
In addition to that, we ensure that the `Postgresql.reset_cluster_info_state()` method is called even if DCS failed (the order of calls was changed in the #1820).
Close https://github.com/zalando/patroni/issues/1981
* 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.
Originally Exhibitor was supported in the ZooKeeper class and
configuration for Exhibitor was taken also from `zookeeper` section in
the yaml config file. In fact, Exhibitor just extends ZooKeeper and now
it is reflected in the code and also Exhibitor got it's own section in
the config.yaml file. It will make it easier to configure Exhibitor
hosts and port via environment variables when PR#211 will be merged.
