Improved HTTP timeout logic

When a client uploads (HTTP POST) a large file there has been a less-than-ideal timeout logic in uWS, where practically 0.1 byte/sec was enough to keep the connection open. This version bumps the required upload throughput to 16kB/sec as the lowest allowed. Any client uploading slower, will be dropped. You can configure the constant HTTP_RECEIVE_THROUGHPUT_BYTES as you wish.