Support for ESP32 and 'message crackers'

The `WiFiClient` of ESP32 has non-standard behavior. When the
(TCP/IP) connection is closed, it will start returning 0 as the result
of `available()`, even though not all octets were read, and one can
actually still `read()` them. So, we need "a middle-man" to keep track
of how many octets are actually available for reading.

This is, unfortunately, a slightly incompatible change. You need to
use the new class `PubNonSubClient` for this to work.  So, even though
you don't _have_ to (and then ESP32 will not work, but it didn't work
either in v2), it is different and warrants a major version bump.

Also, for ease of use, we introduced "message crackers", which should
inspire users to avoid the old `Client` based interface, making it
easier to do similar upgrades/changes in the future.

Examples were also cleaned-up:

* Use new high-level API (message crackers)
* Use ArduinoJson library in the Josn example
* Update comments
* Remove not-very-useful examples
* Reformat with Clang format

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