Add asynchronous downloader that reuses connections to CCDB

[TrifleMichael]

This change adds a downloader capable of parallel and asynchronous curl calls, with the ability to make callback functions after a request is finished.

A lot of the code is base on an example from the curl manual which combines libuv with libcurl (https://curl.se/libcurl/c/multi-uv.html)

The main differences between the example and this code are:

• a mechanism that closes sockets after a set period of time,
• the loop runs a parallel thread to the rest of the program.

In case of any questions or remarks please contact me at [email protected]