io: refactor and improve HTTP modules.

io: refactor and improve HTTP modules.

 - 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)