Pageview and batch/retry updates

Automatic tracking of the mp_page_view event is now off by default. This event has been deprecated for years and is dropped by the Mixpanel API servers.

The batch_requests config option now defaults to true for a subset of projects, turning on batch/queue/retry support. To force this mode on or off explicitly, use the {batch_requests: true} option in mixpanel.init(). The default-on configuration will be gradually rolled out to all projects.