Improve fence handling to prevent anomalies

1. Move fence monitoring to its own thread rather than doing the listing
and triggering within the main keepalive thread.
2. Add a global lock key at /config/fence_lock and use this lock key to
prevent multiple nodes from trying to run fences simultaneously.
3. Run the fencing monitor for each node sequentially within the context
of the main fence monitoring thread, to ensure that fences of multiple
nodes happen sequentially rather than in parallel.

All of these should help to prevent any anomalies where one node can try
to fence multiple nodes at once without recourse.

daemon-common/zkhandler.py

@@ -576,7 +576,7 @@ class ZKHandler(object):
 #
 class ZKSchema(object):
     # Current version
-    _version = 14
+    _version = 15
 
     # Root for doing nested keys
     _schema_root = ""
@@ -592,6 +592,7 @@ class ZKSchema(object):
             "schema.version": f"{_schema_root}/schema/version",
             "config": f"{_schema_root}/config",
             "config.maintenance": f"{_schema_root}/config/maintenance",
+            "config.fence_lock": f"{_schema_root}/config/fence_lock",
             "config.primary_node": f"{_schema_root}/config/primary_node",
             "config.primary_node.sync_lock": f"{_schema_root}/config/primary_node/sync_lock",
             "config.upstream_ip": f"{_schema_root}/config/upstream_ip",