title: Confused about Airflow's BaseSensorOperator parameters : timeout, poke_interval and mode
body: <p>I have a bit of confusion about the way <code>BaseSensorOperator</code>'s parameters work: <code>timeout</code>  &amp; <code>poke_interval</code>.
  Consider this usage of the sensor :</p>
  <pre><code>BaseSensorOperator(
    soft_fail=True,
    poke_interval = 4*60*60,  # Poke every 4 hours
    timeout = 12*60*60,  # Timeout after 12 hours
  )
  </code></pre>
  <p>The documentation mentions the timeout acts to set the task to 'fail' after it runs out. But I'm using a <code>soft_fail=True</code>, I don't think it retains the same behavior, because I've found the task failed instead of skipping after I've used both parameters <code>soft_fail</code> and <code>timeout</code>.</p>
  <p>So what does happen here?</p>
  <ol>
  <li>The sensor pokes every 4 hours, and at every poke, will wait for the duration of the timeout (12 hours)?</li>
  <li>Or does it poke every 4 hours, for a total of 3 pokes, then times out?</li>
  <li>Also, what happens with these parameters if I use the mode=&quot;reschedule&quot;?</li>
  </ol>
  <p>Here's the documentation of the BaseSensorOperator</p>
  <pre><code>class BaseSensorOperator(BaseOperator, SkipMixin):
      &quot;&quot;&quot;
      Sensor operators are derived from this class and inherit these attributes.
      Sensor operators keep executing at a time interval and succeed when
      a criteria is met and fail if and when they time out.
      :param soft_fail: Set to true to mark the task as SKIPPED on failure
      :type soft_fail: bool
      :param poke_interval: Time in seconds that the job should wait in
          between each tries
      :type poke_interval: int
      :param timeout: Time, in seconds before the task times out and fails.
      :type timeout: int
      :param mode: How the sensor operates.
          Options are: ``{ poke | reschedule }``, default is ``poke``.
          When set to ``poke`` the sensor is taking up a worker slot for its
          whole execution time and sleeps between pokes. Use this mode if the
          expected runtime of the sensor is short or if a short poke interval
          is requried.
          When set to ``reschedule`` the sensor task frees the worker slot when
          the criteria is not yet met and it's rescheduled at a later time. Use
          this mode if the expected time until the criteria is met is. The poke
          inteval should be more than one minute to prevent too much load on
          the scheduler.
      :type mode: str
      &quot;&quot;&quot;
  </code></pre>