range_streamer: Use float for time took for stream
range_streamer: Use float for time took for stream

It is useful when the total time to stream is small, e.g, 2.0 seconds
and 2.9 seconds. Showing the time as interger number of seconds is not
accurate in such case.

Message-Id: <b9bea1a51e2a7bf854638ac9906cfd0a30907243@scylladb.com>

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