Add an idle timeout option

[CrowdHailer]

Connections that are opened to Ace, but never receive data should be closed after an certain timeout.
This is the idle_timeout (start_line_timeout).

This would be used to mitigate https://en.wikipedia.org/wiki/Slowloris_(computer_security)
Although this is a much smaller issue normal because of the way erlang handles IO.

• check cowboy option names, try and use recognised terms
• have a start_timeout headers_timeout

We used this python script to test connections

import socket
import time
import select


def check_connection(timeout):
    conn = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    conn.connect(('127.0.0.1', 4100))
    time.sleep(timeout)

    try:
        ready_to_read, ready_to_write, in_error = select.select([conn,], [conn,], [], 5)
    except select.error:
        conn.shutdown(2)    # 0 = done receiving, 1 = done sending, 2 = both
        conn.close()
        print("Connection failed after %ss wait" % timeout)

    conn.sendall("""GET /sys/ping HTTP/1.1\r\nHost: merchant\r\nConnection: keep-alive\r\n\r\n""")
    r = conn.recv(1024)
    if """{"status":"ok"}""" in r:
        print("Connection successful after %ss wait" % timeout)
    else:
        print("Connection failed after %ss wait" % timeout)


if __name__ == "__main__":
    for timeout in [1, 4, 7]:
        check_connection(timeout)