Bootstrapping WordPress triggers multiple PHP warnings and plugin load failures

When CiviCRM cron jobs run in WordPress Multisite, large numbers of warnings are written to the error log because a number of critical objects are not in global scope when WordPress is bootstrapped. The variables in question are $current_site, $current_blog and $current_user. Depending on the level of logging on a server and the plugins present, the log files can completely fill a disk in a very short time.

Of course, the severity of the impact on the logs is proportional to the number of plugins (and the logging level) - but the impact is wider than that because many plugins rely on these objects for their functionality in WordPress Multisite. Some plugins such as bbPress throw vast numbers of warnings - but this affects many other other plugins, some of which happen to handle the issue better than others.

PR to follow.