DS18S20 sensors support simultaneous reading (not if parasitic
powered). This speeds up reading of multiple sensors massively, as the
lengthy conversion is done by all sensors at the same time.
But it does not work unless explicitly started. Also we don't want to
re-start the procedure after each sensor, as that would be as slow as
reading one by one.
Thus, we add a global shared timestamp that gets set once we read the
first sensor. After that, we have 10 second time to read the rest of
the sensors until we set it again. This should be more than enough
time to read large number of sensors in very short time doing only one
parallel conversion.
Signed-off-by: Timo Kokkonen <timo.t.kokkonen@iki.fi>