Add functionality to "pause" repmgrd

In some circumstances, e.g. while performing a switchover, it is essential
that repmgrd does not take any kind of failover action, as this will put
the cluster into an incorrect state.

Previously it was necessary to stop repmgrd on all nodes (or at least
those nodes which repmgrd would consider as promotion candidates), however
this is a cumbersome and potentially risk-prone operation, particularly if the
replication cluster contains more than a couple of servers.

To prevent this issue from occurring, this patch introduces the ability
to "pause" repmgrd on all nodes wth a single command ("repmgr daemon pause")
which notifies repmgrd not to take any failover action until the node
is "unpaused" ("repmgr daemon unpause").

"repmgr daemon status" provides an overview of each node and whether repmgrd
is running, and if so whether it is paused.

"repmgr standby switchover" has been modified to automatically pause repmgrd
while carrying out the switchover.

See documentation for further details.

repmgr--4.1.sql

@@ -145,6 +145,21 @@ CREATE FUNCTION unset_bdr_failover_handler()
   AS 'MODULE_PATHNAME', 'unset_bdr_failover_handler'
   LANGUAGE C STRICT;
 
+CREATE FUNCTION get_repmgrd_pid()
+  RETURNS INT
+  AS 'MODULE_PATHNAME', 'get_repmgrd_pid'
+  LANGUAGE C STRICT;
+
+CREATE FUNCTION get_repmgrd_pidfile()
+  RETURNS TEXT
+  AS 'MODULE_PATHNAME', 'get_repmgrd_pidfile'
+  LANGUAGE C STRICT;
+
+CREATE FUNCTION set_repmgrd_pid(INT, TEXT)
+  RETURNS VOID
+  AS 'MODULE_PATHNAME', 'set_repmgrd_pid'
+  LANGUAGE C STRICT;
+
 
 CREATE VIEW repmgr.replication_status AS
   SELECT m.primary_node_id, m.standby_node_id, n.node_name AS standby_name,