Fix use-after-free race condition in winhttp.

Essentially, the request_context could call ->finish() (and free itself) before the final WinHttp callback completed.

The fix, without changing lifetime semantics, is to use a heap-allocated weak_ptr as the context. This weak_ptr will be freed by the callback function when called with WINHTTP_CALLBACK_STATUS_HANDLE_CLOSING.

Author: ras0219-msft