Use CLOCK_MONOTONIC instead of Time.new to determine idle_since in the Session class

[dazuma]

Google::Cloud::Spanner::Session#idle_since? uses an internal variable @last_updated_at to track the last change timestamp. This object is currently a Time object retrieved via Time.now, but it is never accessed directly, only in comparison with other Time objects to determine durations. For this purpose, it is better practice to use Process::clock_gettime(Process::CLOCK_MONOTONIC) which isn't sensitive to changes in the clock (e.g. due to time server updates).

We should also audit the rest of the code base to ensure there aren't other cases of this.