Fix compatibility with WP Cron Control plugin

[fjarrett]

A user recently reported that Stream was not logging any data when the WP Cron Control plugin was also present and activated.

It turns out that this plugin overrides the DOING_CRON constant which we depend on for determining the current user agent.

Since WP Cron Tracking is disabled by default in Stream's Advanced settings tab, no activity is ever tracked because it assumes every action is being performed by a cron job.

The quick fix for this is to enable WP Cron Tracking, but a more complete fix is still needed for full compatibility with this plugin.