Add SolrCore.withSearcher(lambda accepting SolrIndexSearcher)

  /**
   * Executes the lambda with the {@link SolrIndexSearcher}.  This is more convenience than using
   * {@link #getSearcher()} since there is no ref-counting business to worry about.
   * Example:
   * <pre class="prettyprint">
   *   IndexReader reader = h.getCore().withSearcher(SolrIndexSearcher::getIndexReader);
   * </pre>
   */
  @SuppressWarnings("unchecked")
  public <R> R withSearcher(Function<SolrIndexSearcher,R> lambda) {
    final RefCounted<SolrIndexSearcher> refCounted = getSearcher();
    try {
      return lambda.apply(refCounted.get());
    } finally {
      refCounted.decref();
    }
  }