Add missing Admin mutex locking during server discovery

Right now we lock on Admin everytime we are adding discovered servers.
This prevents concurrent configuration modifications, and also ensures
consistency during configuration promotions. In the future this locking
*may become* not necessary.

lib/MySQL_HostGroups_Manager.cpp

@@ -8595,6 +8595,8 @@ void MySQL_HostGroups_Manager::add_discovered_servers_to_mysql_servers_and_repli
 	const vector<tuple<string, int, int>>& new_servers
 ) {
 	int added_new_server = -1;
+
+	GloAdmin->mysql_servers_wrlock();
 	wrlock();
 
 	// Add the discovered server with default values
@@ -8657,4 +8659,5 @@ void MySQL_HostGroups_Manager::add_discovered_servers_to_mysql_servers_and_repli
 	}
 
 	wrunlock();
+	GloAdmin->mysql_servers_wrunlock();
 }