Argument "--type [callback,wait]" is added to be able to switch between
"wait_for_event()" (previous functionality) and "event_callback()" in
the I2C slave functionality.
In a thread set_callback() executes event_callback() and just (very
dirty) sleeps 2 seconds to get a response or else cancels the event and
stops pigpio.
To get the most reliable output pigpiod is started with -s 2 (sample
rate 2 microseconds). Still most of the time too few bytes are received
on the "getdatalog" action (153 bytes expected).
