net: coap: retransmission for four times
net: coap: retransmission for four times

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 <563fc56b80720efb29939356d471536b8c17d514@acer.com>

GiulianoFranchetto/zephyr,explora26/zephyr,finikorg/zephyr,zephyriot/zephyr,mbolivar/zephyr,nashif/zephyr,nashif/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,finikorg/zephyr,zephyriot/zephyr,ldts/zephyr,punitvara/zephyr,punitvara/zephyr,kraj/zephyr,punitvara/zephyr,galak/zephyr,Vudentz/zephyr,zephyriot/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,punitvara/zephyr,galak/zephyr,kraj/zephyr,Vudentz/zephyr,zephyriot/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,ldts/zephyr,kraj/zephyr,Vudentz/zephyr,explora26/zephyr,mbolivar/zephyr,Vudentz/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,galak/zephyr,mbolivar/zephyr,explora26/zephyr,kraj/zephyr,mbolivar/zephyr,nashif/zephyr,mbolivar/zephyr,explora26/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,Vudentz/zephyr,kraj/zephyr,Vudentz/zephyr,zephyriot/zephyr