- refactor code used multiple times spread over sg/fg into
one single location.
- allow aborting requests.
- Provide two common request types:
* FileRequest: Save response into file
* MemoryRequest: Keep resonse in memory (std::string)
- extend HTTP::Client interface:
* urlretrieve: Save url to file (shortcut for making a
FileRequest)
* urlload: Get respons into memory (shortcut for making
a MemoryRequest)