Use ESP8266 SDK SNTP to fix "benchmark" loop

[3rwww1]

estimate: 2j

The goal is to use the SDK library instead of our own "home-brew" NTP implementation, which has several problems:

• the NTP "benchmark" is useless and causes an infinite loop, where a coolboard that has had only one wifi with no no internet configured never can start an AP again without erasing the SPIFFS (@simon-lacool, told you this was a bad idea...)
• using the SDK is so much better anyway: less code to maintain, probably more reliable, etc.

The approaches differ a lot:

• COOL approach: connect to Wifi, make NTP requests until we get an answer, or timeout (and, currently, we do not handle it correctly and let Bad Things© happen, such as dates in 2066, etc)
• SDK approach. Basically, it runs down to "connect to wifi and at some point, you'll automagically be sync'ed with NTP".

Those differences create numerous caveats:

• the way of finding out if the NTP sync was done recently is much more complex
• handling offline or nomad connections, offline logging, etc, is tedious and not well documented
• the SDK aims at providing timezone, DST aware date and time, and it is really not our use case.
• the documentation is lacking and mostly in github issues conversations

Way I see it, the algorithm should look like this:

• initialize a class variable bool ntpSynced = false in CoolBoard.cpp or CoolTime.cpp
• configure a callback as seen in SDK example. it will be called on NTP sync success.
• connect to wifi
• (lots of code happening)
• before sending the first log, wait max NTP_SYNC_TIMEOUT seconds (I think 10s) for ntpSynced to become true
• if ntpSync is true: send MQTT messages, as we are certain the timestamp is correct
• if ntpSync is false: bail out, as we can't handle anything good at this point before an RTC enhancement and a good error reporting funnel

Must reads:

• Sole example : https://github.com/esp8266/Arduino/blob/master/libraries/esp8266/examples/NTP-TZ-DST/NTP-TZ-DST.ino (and we're not interested in the TZ and DST parts)
• Discussions : here and here