lwip_sock struct

sockets currently are built on netconns, each socket has one netconn.

data that was left from the previous read.

number of times data was received, set by event_callback(), tested by the receive and select functions.

number of times data was ACKed (free send buffer), set by event_callback(), tested by select.

error happened for this socket, set by event_callback(), tested by select.

counter of how many threads are waiting for this socket using select.