Change master->primary/leader/member (#2541)

keep as much backward compatibility as possible.

Following changes were made:
1. All internal checks are performed as `role in ('master', 'primary')`
2. All internal variables/functions/methods are renamed
3. `GET /metrics` endpoint returns `patroni_primary` in addition to `patroni_master`.
4. Logs are changed to use leader/primary/member/remote depending on the context
5. Unit-tests are using only role = 'primary' instead of 'master' to verify that 1 works.
6. patronictl still supports old syntax, but also accepts `--leader` and `--primary`.
7. `master_(start|stop)_timeout` is automatically translated to `primary_(start|stop)_timeout` if the last one is not set.
8. updated the documentation and some examples

Future plan: in the next major release switch role name from `master` to `primary` and maybe drop `master` altogether.
The Kubernetes implementation will require more work and keep two labels in parallel. Label values should probably be configurable as described in https://github.com/zalando/patroni/issues/2495.

postgres0.yml

@@ -51,7 +51,7 @@ bootstrap:
     loop_wait: 10
     retry_timeout: 10
     maximum_lag_on_failover: 1048576
-#    master_start_timeout: 300
+#    primary_start_timeout: 300
 #    synchronous_mode: false
     #standby_cluster:
       #host: 127.0.0.1