Avoid blocking in NTPClient::update()

[mlesniew]

When a NTP request is sent, it may take several milliseconds to retrieve the response. This commit changes the NTPClient::update() behaviour to asynchronous allowing a NTP request to be sent with one update() call and handle the response when it's available in another call, eliminating active waiting.

This commit also changes the NTPClient::forceUpdate() implementation to rely on the logic in NTPClient::update(). However, the behaviour of this function does not change from the API user's perspective. It is still synchronous, it only returns when all processing is complete.

This fixes issue #112.

Note that this is equivalent to what was suggested in PR https://github.com/arduino-libraries/NTPClient/pull/90. I decided to reimplement it, because the other PR seems to be forgotten by the author and he still hasn't signed the CLA.

[CLAassistant]

All committers have signed the CLA.

[github-actions[bot]]

Memory usage change @ 3f9957dcf14cc3d6a8250ba86f5f686b87492a0b

Board	flash	%	RAM for global variables	%
esp8266:esp8266:huzzah	:green_heart: -64 - -64	-0.01 - -0.01	:small_red_triangle: +8 - +8	+0.01 - +0.01

Click for full report table

Board|examples/Advanced
flash|%|examples/Advanced
RAM for global variables|%|examples/Basic
flash|%|examples/Basic
RAM for global variables|%|examples/IsTimeSet
flash|%|examples/IsTimeSet
RAM for global variables|% -|-|-|-|-|-|-|-|-|-|-|-|- esp8266:esp8266:huzzah|-64|-0.01|8|0.01|-64|-0.01|8|0.01|-64|-0.01|8|0.01

Click for full report CSV

``` Board,examples/Advanced
flash,%,examples/Advanced
RAM for global variables,%,examples/Basic
flash,%,examples/Basic
RAM for global variables,%,examples/IsTimeSet
flash,%,examples/IsTimeSet
RAM for global variables,% esp8266:esp8266:huzzah,-64,-0.01,8,0.01,-64,-0.01,8,0.01,-64,-0.01,8,0.01 ```

[AmirHmZz]

Any updates on this?

[mlesniew]

I believe this PR is ready to be merged, we're just waiting for one of the maintainers to accept it.