Espurna fixes + backports

[mcspr]

• Check if received timestamp is 0 before converting it to unix ts. Rare case when NTP server hates us, because we are sending too much requests / broken / unsynchronized / does not return correct data / (?) wifiudp returns bad data e.g. ntp timestamp is zero and -70y results in overflow:

>>> datetime.datetime.fromtimestamp((2 ** 32) - 2208988800)
datetime.datetime(2036, 2, 7, 9, 28, 16)

• Additional check for stratum value (per https://tools.ietf.org/html/rfc5905#page-20)
• Do not recreate WiFiUDP instance when calling .begin()
• Minor typing update: udp.read() returns binary data, not char string

edit:

• New check for receive and reference timestamps, kind-of sanity check for the NTP server

edit2:
ezTime checks with slightly shorter bitshift. ezTime itself is not here yet, pending some tweaks to make timezone handling more robust. until then - manually tweaking NtpClient

[mcspr]

Ping.
Ofc it might be the best idea to actually "vendor" this version inside espurna repo. I think I start to understand ESPEasy / Tasmota way of things