Add dcs_last_seen field to API (#2051)

This field notes the last time (as unix epoch) a cluster member has successfully communicated with the DCS. This is useful to identify and/or analyze network partitions.

Also, expose dcs_last_seen in the MemberStatus class and its from_api_response() method.

tests/test_api.py

@@ -54,6 +54,7 @@ class MockHa(object):
 
     state_handler = MockPostgresql()
     watchdog = MockWatchdog()
+    dcs_last_seen = 0
 
     @staticmethod
     def is_leader():
@@ -77,7 +78,7 @@ class MockHa(object):
 
     @staticmethod
     def fetch_nodes_statuses(members):
-        return [_MemberStatus(None, True, None, 0, None, {}, False)]
+        return [_MemberStatus(None, True, None, 0, 0, None, {}, False)]
 
     @staticmethod
     def schedule_future_restart(data):

tests/test_ha.py

@@ -80,13 +80,14 @@ def get_standby_cluster_initialized_with_only_leader(failover=None, sync=None):
     )
 
 
-def get_node_status(reachable=True, in_recovery=True, timeline=2,
-                    wal_position=10, nofailover=False, watchdog_failed=False):
+def get_node_status(reachable=True, in_recovery=True, dcs_last_seen=0,
+                    timeline=2, wal_position=10, nofailover=False,
+                    watchdog_failed=False):
     def fetch_node_status(e):
         tags = {}
         if nofailover:
             tags['nofailover'] = True
-        return _MemberStatus(e, reachable, in_recovery, timeline, wal_position, tags, watchdog_failed)
+        return _MemberStatus(e, reachable, in_recovery, dcs_last_seen, timeline, wal_position, tags, watchdog_failed)
     return fetch_node_status