[Do not merge] Rapidjson

[Cylix]

This is a first draft of using rapidjson.

Please do not merge:

• I first want to make more testing. First, because the change is critical as it concerns the requests themselves, but also because this library has a very different memory management (which makes it powerful but also may need to be cautious), and finally because I want to make sure this change is worth it.
• I believe this change would be worth it with some additional changes in the requests/responses design that would avoid copies. I believe if we go in the direction of rapidjson in order to reduce memory footprint (that's the whole point of that change), something that would be even more interesting is to make the requests more as a temporary buffer that can be move to the json memory pool. Same thing concerning the json itself by moving the json buffer into the response itself. This will require more thinking though.

This PR mostly opened to keep track of what is being done, making suggestions, think through and evaluate the cost of the change based on our library goals.

It is related to #252 and aims to reduce memory footprint.