Refactor server version detection

Most of the time we can simply get the version number directly from
the connection handle. Previously it was held in a global variable,
which was an icky way of doing things.

In a few special cases we also need the actual version string, which
is obtained directly from the database.

repmgrd.c

@@ -372,12 +372,6 @@ main(int argc, char **argv)
 	/* abort if local node not available at startup */
 	local_conn = establish_db_connection(config_file_options.conninfo, true);
 
-	/*
-	 * store the server version number - we'll need this to generate
-	 * version-dependent queries etc.
-	 */
-	server_version_num = get_server_version(local_conn, NULL);
-
 	/*
 	 * sanity checks
 	 *