Advance Shard Downloader

[undertome]

Add the following features to the SSLHTTPDownloader:

• The ability to pause/resume downloads.

• The ability to resume downloads after a software crash.

• (Optional) The ability to download from multiple servers to a single file.

[undertome]

Two options for interrupting the file download:

• Override the on_body_impl method of the response_parser class. This method is called whenever the parser receives some data.

• Change the download method from async_read to async_read_some. Place the latter in a loop, and in between calls, just check the isPaused_ member variable.

[undertome]

Two options for interrupting the file download:

• Override the on_field_impl method of the response_parser class. This method is called whenever the parser receives some data.
• Change the download method from async_read to async_read_some. Place the latter in a loop, and in between calls, just check the isPaused_ member variable.

Looks like the override method won't work. on_body_impl is private, so we can't call it after doing our stuff in an overridden method, and the method body accesses private members so we can't just duplicate the logic. Same for on_chunk_body_impl.

[undertome]

Testing to see whether multiple instances of handler cause issues. Should there be stronger enforcement of singleton use?