Fixes a GCC warning because %llu appears to be unsupported by the
Windows standard library.

Fixes a GCC warning because %llu appears to be unsupported by the
Windows standard library.

Eliminate dependency on io_thread_get().

Call SeekDone() to avoid the freeze bug.

This allows constructing an instance in any thread, and register it
inside the IOThread later.

The CurlRequest gives us a more low-level API to CURL without the
InputStream interface, integrated into our IOThread.

Simplifies the code and reduces the number of IOThread roundtrips.

default_value defaults to nullptr.

Fixes deadlock on small responses.

The old struct config_param remains only for top-level string options.