repmgr: initialise "voting_term" in "repmgr primary register"

This previously happened in the extension SQL code, which could
potentially cause replay problems if installing on a BDR cluster.

As this table is only required for streaming replication failover,
move the initialisation to "repmgr primary register".

Addresses GitHub #344 .

dbutils.h

@@ -473,6 +473,7 @@ bool		delete_monitoring_records(PGconn *primary_conn, int keep_history);
 
 
 /* node voting functions */
+void		initialize_voting_term(PGconn *conn);
 int			get_current_term(PGconn *conn);
 void		increment_current_term(PGconn *conn);
 bool		announce_candidature(PGconn *conn, t_node_info *this_node, t_node_info *other_node, int electoral_term);