rss2i.py: laxTimeout needed for stopping measurements

Since the timings for aborting a measurement session changed (see dc8100df), there may occur serial timeouts when executing

rss2i -a stop

In order to circumvent this, we should allow more lax timeouts for the stop command. The laxTimeout times were already fitted (see b6ef535e) to the changed measurement timings. Requesting those laxTimeouts, as it is done e.g. for the download command, needs still to be done.