BUG: refs #974. Change "large" file threshold for zipstreaming

This makes it much less likely that a folder with a huge number of items will
cause out of memory crashes.  It could still happen though; thus is the nature of
zip streaming.

library/ZipStream/ZipStream.php

@@ -155,9 +155,9 @@ function __construct($name = null, $opt = array()) {
     # save options
     $this->opt = $opt;
 
-    # set large file defaults: size = 20 megabytes, method = store
+    # set large file defaults: size = 3 megabytes, method = store
     if (!isset($this->opt['large_file_size']))
-      $this->opt['large_file_size'] = 20 * 1024 * 1024;
+      $this->opt['large_file_size'] = 3 * 1024 * 1024;
     if (!isset($this->opt['large_file_method']))
       $this->opt['large_file_method'] = 'store';