* Fix bug in sntp where callback was run at interrupt level. Also add
the nodemcu pool servers as the default.
* Add comments on where the mysterious numbers came from
* Fix a crash with auto repeat mode and errors on repeat
* Provide an error code to SNTP error callback.
* Switch SNTP to use ephemeral port.
In case we're being hit by ISP-level thou-shall-not-run-NTP silliness.