Pass a long. The set number will be used as the maximum amount of
simultaneously open connections that libcurl may cache. Default is 10, and
libcurl will enlarge the size for each added easy handle to make it fit 4
times the number of added easy handles.
By setting this option, you can prevent the cache size to grow beyond the
limit set by you.
When the cache is full, curl closes the oldest one in the cache to prevent the
number of open connections to increase.
This option is for the multi handle's use only, when using the easy interface
you should instead use the CURLOPT_MAXCONNECTS option.
(Added in 7.16.3)