Allow developers to disable the swapping of gq params for q.alt params

There's a section in apachesolr_do_query() that removes fq params, and moves them into a q.alt statement, if certain conditions match.

  $keys = $query->getParam('q');
  if (strlen($keys) == 0 && ($filters = $query->getFilters())) {
    // Move the fq params to q.alt for better performance. Only suitable
    // when using dismax or edismax, so we keep this out of the query class itself
    // for now.
    $qalt = array();
    foreach ($filters as $delta => $filter) {
      // Move the fq param if it has no local params and is not negative.
      if (!$filter['#exclude'] && !$filter['#local']) {
        $qalt[] = '(' . $query->makeFilterQuery($filter) . ')';
        $query->removeFilter($filter['#name'], $filter['#value'], $filter['#exclude']);
      }
    }
    if ($qalt) {
      $query->addParam('q.alt', implode(' ', $qalt));
    }
  }

When it comes to scoring and boost biasing, fq and q.alt are treated differently: filters in fq do not contribute to the score of retrieved documents, whilst filters in q.alt do. For example, if the filter was "color=red OR color=blue", and red comprised 1% of the total documents whilst blue comprised 10% of the total documents, the filter would give a higher score to red documents if it were set using q.alt. If you wanted the filters to behave neutrally, and bias the results in a different way using a custom boost function, then the filter conditions must stay as fq.

This patch adds an additional clause, so that developers can disable the performance-optimization via an environment variable.