net: coap: retransmission for four times

RFC 7252, sec 4.8 [https://tools.ietf.org/html/rfc7252#section-4.8

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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>