RFC 7252, sec 4.8 [https://tools.ietf.org/html/rfc7252#section-4.8]
The default MAX_RETRANSMIT is 4, current implementation only
retry 3 times. Update next_timeout() to return one more retry.
Also, add a TODO for random generated initial ACK timeout
Signed-off-by: Robert Chou <robert.ch.chou@acer.com>