Make Ultrasonic plugin running async.

[TD-er]

A number of fixes were done in this PR: #4166

Right now a ping call is waiting for the result and thus blocking.

We need to make this event driven, so the task will continue when the ping reply has been received.
However this makes things a bit more complex when having several ultrasonic tasks as you don't want several pings to be "in transit" or else the sensor may have no idea if the reflected signal is a result from its sent out ping or an echo from another sensor.

Still to do:

• Check if several ping tasks can send out pings while other is still active
• Actually test on ESP32 (and ESP8266) to get some idea of the improvements using direct GPIO access
• Document plugin
• Test several Ultrasonic sensor(s)