function issueRequest(request) {
  if (request.state === RequestState.UNISSUED) {
    request.state = RequestState.ISSUED;
    request.deferred = defer();
  }
  return request.deferred.promise;
}
Check if the priority heap has open slots, regardless of which server they
are from. This is used in {@link Multiple3DTileContent} for determining when
all requests can be scheduled
@param {number} desiredRequests The number of requests the caller intends to make
@return {boolean} <code>true</code> if the heap has enough available slots to meet the desiredRequests. <code>false</code> otherwise.

@private