Identity management updates

The mixpanel.identify() implementation has been updated for compatibility with Mixpanel's new identity management system (v3). From this version, we will prefix randomly-generated device-specific distinct_ids with "$device:". The prefix is applied the next time a new random ID is generated; any IDs generated by previous SDK versions and persisted in the browser will continue to be used as-is until reset is called to generate a new ID. This does not change the value sent for the $device_id property, which will continue to be the randomly-generated ID without a prefix. Mixpanel's $identify endpoint has been updated to accept UUIDs with this prefix to coordinate with this change.

This release also contains more aggressive client-side deduplication in the event-batching system, to reduce superfluous network sends in edge cases where parts of the queue/batch system fail. Related to this update, events now include a property mp_sent_by_lib_version which can distinguish the version of the library that actually sent an event over the network vs the version that originally queued the event.