Zabbix

........S. [ZBXNEXT-3877] Use the curl_multi_* API instead of the curl_easy_* API

The use of the multi api can allow the server to handle multiple connections in
parallel in a non-blocking way.
This translate in a performance improvement during data sending to the history
service in the order of more than 5X.
We first prepare all the CURL handles that we need with their buffers and URL
ready. After that they are added to the multi handler and once everything is
in place, send the data in parallel.
More optimization can be performed here, probably we can reduce the
amount of connections used by pipelining the requests in the handler.
This patch also generalize the value processing, by being transparent on
how many handlers are created. All the important informations are packed
in the senders array of structures and adding a new value type in the future
require changing just some minor places.

Idea from Andris Z. about the usage of the multi api.