Add packet validity checks

[bmcdonnell]

Addresses #133. Improves upon #28 (not merged).

[CLAassistant]

All committers have signed the CLA.

[github-actions]

Memory usage change @ cb073ba

Board	flash	%	RAM for global variables	%
esp8266:esp8266:huzzah	🔺 +112 - +112	+0.01 - +0.01	0 - 0	0.0 - 0.0

Click for full report table

Board	examples/Advanced flash	%	examples/Advanced RAM for global variables	%	examples/Basic flash	%	examples/Basic RAM for global variables	%
esp8266:esp8266:huzzah	112	0.01	0	0.0	112	0.01	0	0.0

Click for full report CSV

Board,examples/Advanced<br>flash,%,examples/Advanced<br>RAM for global variables,%,examples/Basic<br>flash,%,examples/Basic<br>RAM for global variables,%
esp8266:esp8266:huzzah,112,0.01,0,0.0,112,0.01,0,0.0

[bmcdonnell]

I added a check of the fractional part of the reference time, since per RFC4330,

There will exist a 232-picosecond interval, henceforth ignored, every 136 years when the 64-bit field will be 0, which by convention is interpreted as an invalid or unavailable timestamp.

This time does roll over, so it can hit zero again. The reference time isn't updated frequently - it's not always getting updated in subsequent queries like the other timestamps - so this force push covers a very rare but still conceivable corner case (vs the previous revision).

[bmcdonnell]

Marked as draft because I'm getting some NTP version 3 responses, which this was rejecting.

[github-actions]

Memory usage change @ 38a7dff

Board	flash	%	RAM for global variables	%
esp8266:esp8266:huzzah	🔺 +160 - +160	+0.02 - +0.02	0 - 0	0.0 - 0.0

Click for full report table

Board	examples/Advanced flash	%	examples/Advanced RAM for global variables	%	examples/Basic flash	%	examples/Basic RAM for global variables	%
esp8266:esp8266:huzzah	160	0.02	0	0.0	160	0.02	0	0.0

Click for full report CSV

Board,examples/Advanced<br>flash,%,examples/Advanced<br>RAM for global variables,%,examples/Basic<br>flash,%,examples/Basic<br>RAM for global variables,%
esp8266:esp8266:huzzah,160,0.02,0,0.0,160,0.02,0,0.0