#23 [PHP]Add connecting pool

[Jira Trac]

PHP driver will establish a new connect when you use cubrid_connect or cubrid_connect_with_url. But when we use the same parameters in the same request, we should use the connect already exist. PHP driver will establish a new connect in this situation. Establishing a connect is consumption of resources, so we add connecting pool to handle connections, that is, when you establish a connection use the same parameters that you have used before in the same request, it won't establish a new connect, but use the connect already existing in the connecting pool.