detect slow send buffer drainging pace using TCP_INFO

[Antoine Martin]

See ​When TCP sockets refuse to die:

An example of checking the draining pace:

while True:
    notsent1 = get_tcp_info(c).tcpi_notsent_bytes
    notsent1_ts = time.time()
    ...
    poll.poll(POLL_PERIOD)
    ...
    notsent2 = get_tcp_info(c).tcpi_notsent_bytes
    notsent2_ts = time.time()
    pace_in_bytes_per_second = (notsent1 - notsent2) / (notsent2_ts - notsent1_ts)
    if pace_in_bytes_per_second > 12000:
        # pace is above effective rate of 96Kbps, ok!
    else:
        # socket is too slow...

Using ​TCP_INFO.

See also #2420