title: mat-slider change event triggered too often
body: <p>I'm using a mat-slider to let the user set a value.
  The <code>(input)</code> event emitter is used for displaying the value "live" (while mouse still down).
  This is working.
  At the moment the <code>(change)</code> event emitter is used to recognize when the user has selected a value (released mouse).</p>
  
  <p>The first problem is that the change event is fired twice during a normal interaction with the slider: once on the initial click on the slider and the second time when releasing it.
  I would like to have the event fired only upon releasing the slider, or to have a function which can differentiate between the two events and act only upon the release event.</p>
  
  <p>The second problem is that I have to make an HTTP request when a new value from the slider is set. I don't want this to happen to often, so I would like to have a kind of a timeout. Only when a certain amount of time has passed and no new change event occured, do I want to make the HTTP request.</p>
  
  <p>I already tried to implement this with a lock, but it didn't work because sometimes only one change event is fired (for example when clicking on the slider and not dragging it), and then the lock variable is flipped wrongly and the lock mechanism is not working anymore because of the inverted variable.</p>
  
  <p>Is there any way to resolve these problems and to achieve what I want?</p>