Optimization of libcurl usage (not releasing curl_handle at every request)

Hello, 

Here is an optimization I've made on this libcurl vmod. I've used the ability offered by libcurl to reuse a curl_handle for multiple requests. It allows to keep active connections, DNS cache, etc... , thus improving performances.
Tested/used with varnish 4.0.2

Hope you will find this helpful.
Regards,