Support for Cache Consistent

Thanks for the great module!

We use Cache Consistent → to handle some issues with Memcache. That module require setting the default cache class to 'ConsistentCache' whereas Acquia Purge ONLY allows Memcache to be used if it is set to MemcacheDrupal:

function _acquia_purge_are_we_using_memcached() {
  static $answer;
  if (is_null($answer)) {
    global $conf;
    $answer = FALSE;

    if (_acquia_purge_variable('acquia_purge_memcache')) {
      if (isset($conf['cache_default_class'])) {
        if ($conf['cache_default_class'] === 'MemCacheDrupal') {
          if (function_exists('dmemcache_get')) {
            $answer = TRUE;
          }
        }
      }
    }
  }
  return $answer;
}

Since Cache Consistent seems to be a transparent, txn-aware wrapper around Memcache, it makes sense to also allow this default class to pass the Memcache test. eg.:

 if ($conf['cache_default_class'] === 'MemCacheDrupal' || ($conf['cache_default_class'] === 'ConsistentCache' && $conf['consistent_cache_default_class'] == MemCacheDrupal)) {

I made this adjustment in our dev environment and it worked as expected. This is much more elegant than relying on the file storage default, which is what happens in our case when using the drop-in replacement class provided by Cache Consistent.