LWIP_TCP_MAX_SACK_NUM macro

LWIP_TCP_MAX_SACK_NUM: The maximum number of SACK values to include in TCP segments. Must be at least 1, but is only used if LWIP_TCP_SACK_OUT is enabled. NOTE: Even though we never send more than 3 or 4 SACK ranges in a single segment (depending on other options), setting this option to values greater than 4 is not pointless. This is basically the max number of SACK ranges we want to keep track of. As new data is delivered, some of the SACK ranges may be removed or merged. In that case some of those older SACK ranges may be used again. The amount of memory used to store SACK ranges is LWIP_TCP_MAX_SACK_NUM * 8 bytes for each TCP PCB.