Before this commit, since our diesel heater application uses one line
for RX and TX between the controller and ECU, our MITM device had its
RX1 connected (through the anode of a diode) to TX1, and the same with
TX2. Thus, when open-collector TX1 modulates by pulling the line low,
the esp8266 would also see what it transmitted as a RX byte.
Ignore those bytes by ignoring characters received on each line after
transmit for some time.
As for exactly how long to suppress: here, we try RX_SUPPRESS_INTERVAL
of 50ms. This is pretty application-specific, but ... for our diesel
heater implementation, the controller transmits once a second at most,
and (after having its say) the ECU replies no faster than 200ms later.
