NTP-based internet test

[avtolstoy]

Problem

1. Gen 3 platforms do not have internet test at all
2. Using 8.8.8.8:53 over TCP may not be ideal as it's TCP and most devices nowadays use UDP for cloud comms and it skips DNS step
3. The logic around resetting network interfaces due to a number of failed cloud connection attempts is broken and only works somewhat correctly with TCP-based devices (Photon and P1 only)

Solution

This PR introduces an SNTP (Simple Network Time Protocol) implementation to be used for internet tests. By default we are targeting pool.ntp.org, which allows us to also test DNS and allows us to use the distributed network of NTP servers around the globe in the pool over UDP.

We are also changing the logic around when to trigger network interface reset due to cloud connection errors. Previously only "connection" attempts were registered as failures, whereas handshake errors were not taken into account at all. This does work somewhat ok for TCP-based devices (Photon/P1), but these days does not make much sense for connection-less UDP-based devices.

Now, we are taking into account any kind of a cloud connection error and will wait up to 5 minutes of consecutive cloud connection failures across all platforms to trigger a reset of network interfaces.

Steps to Test

• Unit tests
• Corrupt device/server keys, watch the logs. The device should reset network interfaces after about 5 minutes
• Probably easiest to test on WiFi devices: disconnect WAN on router/set firewall rules to forbid the devices to communicate over UDP. The device should reset network interfaces after about 5 minutes

Example App

N/A

References

• [CH36520]
• [CH48170]
• [CH51505]
• [CH34989]

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Completeness

• User is totes amazing for contributing!
• Contributor has signed CLA (Info here)
• Problem and Solution clearly stated
• Run unit/integration/application tests on device
• Added documentation
• Added to CHANGELOG.md after merging (add links to docs and issues)

[keeramis]

I will start by testing out some tests mentioned on one of my devices.

[sergeuz]

I'm approving this PR since it does the job. The timestamp calculations don't seem to be correct though.