repmgrd: check binary and extension major versions match

repmgr requires that the same "major version" (e.g. 4.3) is present
on all nodes, otherwise - particularly in the case of repmgrd - it's
highly likely things won't work as expected.

Implements part of GitHub #515.

dbutils.h

@@ -112,12 +112,16 @@ typedef enum
 
 typedef struct s_extension_versions {
 	char		default_version[8];
+	int			default_version_num;
 	char		installed_version[8];
+	int			installed_version_num;
 } t_extension_versions;
 
 #define T_EXTENSION_VERSIONS_INITIALIZER { \
 	"", \
+	UNKNOWN_SERVER_VERSION_NUM, \
 	"", \
+	UNKNOWN_SERVER_VERSION_NUM \
 }
 
 /*