Hardening/libcurl

**DON'T MERGE**

Orion uses libcurl to send HTTP requests.
Leting libcurl to eat all the available memory seem to be dangerous... I think we should set some limit and, to be coherent, I think we can use the limits in the pre-libcurl version (I8MB, as said at https://forge.fi-ware.org/plugins/mediawiki/wiki/fiware/index.php/Publish/Subscribe_Broker_-_Orion_Context_Broker_-_User_and_Programmers_Guide#Notification_maximum_size).

In adition, we should have a .test to check that these limits are honored.

What do you think? @kzangeli opinion is also valuable here.